Guys I think I figured it to some extent. I don't think she was on drugs at all, nor was she playing hide and seek. She may have been tired or a bit drunk, but nothing more. She is apparently traveling on her own for the first time, so this increases her chances of attracting the attention of criminals, especially since she is alone and a woman. My guess is she is having problems before going inside.
Please watch the video carefully with my explanations, every small body movement is important.
1- People usually wait in the center between both elevators where the up/down button is. So from that we can assume she had been waiting between both elevators for one to show up, and it's possible that two were relatively close at the same time, otherwise she may have moved more towards the front of the elevator, knowing which one was on its way. Or, she came out from the other elevator and directly into this one.
She was standing there in the middle:
2- (0:06) When she enters, she isn't afraid or panicking. She very casually presses multiple buttons, one after the other, in a downward fashion. There are three rows of buttons. She presses six times, meaning floor 12, 9, 6, 3, Lobby, and Door Open button. There seems to be no logic to choosing those floors.
Now who usually pushes multiple buttons on an elevator? A pranksters would, to force the elevator to waste people's time and go to multiple floors, but they would usually do so when exiting the elevator, not when going in, unless the prankster was going to the next stop and leave it to continue on its way afterwards. This seems very unlikely in this case. My guess? This is NOT the first time she tries to use the elevator. In fact, it's possible she had the same issue with the other elevator on the left side or possibly both, every time she tried to use the elevator at that point in time (not sure what time it is). This makes sense later on (see point 16).
3- As she pushes the last button (Door Open button), the doors are closing, and this causes the doors to re-open. This doesn't startle her, but she doesn't realize it's because she pressed that button.
4- (0:15) Proof again that she isn't afraid at this point: when she is done with the buttons, she stands in the corner of the elevator, but in no way is she hiding, she would be visible in the mirror or to anyone passing in front of the elevator. But soon she wonders why the doors aren't closing.
5- The doors don't close, so she is wondering what's going on. She stares into the mirror in the hallway from inside the elevator, sees there is no one close to the elevator, so she looks out quickly to see further down the hall, both ways (first to her right, which she couldn't see in the mirror from inside). This is NOT the reaction of someone who is deeply afraid, most people would just have hid right away and waited, not look out quickly as if to catch someone in the act. She is in a "confrontational" state, ready to yell at someone messing with her, that's why she looks out with such determination. My belief is that at this point she is "annoyed". She thinks someone is messing around.
6- She sees no one on either sides, and steps back inside standing still in the center, not hiding. A small pause. This pause appears to indicate that at first she saw and heard nothing in the hallway, so she simply resumed to her position waiting for the doors to close, looking in the mirror (0:27), but THEN she hears something in the hall, hence she now becomes afraid or doesn't want to be seen, and she feels that sound is to her right.
7- She moves to the side to hide, but she is looking into the mirror from there and realizes someone could see her in the mirror too, so she then steps into the corner, from where she can see nothing but cannot be seen either. She waits.
8- She decides to look out again. (0:38) Notice how she looks into the mirror first, sees no one, then decides to look further down the hall, focused on what could be to her right, peering out barely. This to me confirms she thinks she heard a sound in the hall to her right and that she has a good view of her left with the mirror. After a very short moment, starring at her right and slightly hidden, (0:46) she slowly turns to her left, as if realizing nothing is to her right, looking into the mirror again, and then after a pause starring in the mirror she decides to step out carefully to look down the left side of the hall, probably thinking the sound might have been in that direction all along.
9- As she peers out, she leans forward fixated in that direction, and suddenly does a little jump forward as if startled (0:53). Note her hands are together during the whole time, even during the jump. It doesn't seem to be her trying to spook someone by leaping forward. She has made her startled-like leap, her hands are still together, and she is still looking to her left. As if her feeling of being startled was interrupted itself as she jumped forward, as if not sure what to feel (frightened, reassured at the same time).
10-
Look very carefully at that point in the video! She has made eye contact with someone who is moving in the hallway from the left side (coming from the left corner after the other elevator), and that person moves toward her or the elevator. She is looking at someone moving. Straightens her back, opens her legs, and looks slightly more forward but still at an angle, tracking the person moving probably towards her. This is a very "stable" position. That's why her startled jump was "interrupted" (she didn't have her hands let go and straightened herself quickly), she saw someone and her brain wondered if the person is a threat or not.
11- (0:57) The person moves closer, and now she closes her legs again but by placing herself towards the left side, taking a more normal standing position to talk with the person to her left. The person appears to be right in front of where the elevator buttons would be.
12- (1:00) She steps back in the elevator, a bit afraid, but almost immediately steps out again, probably as the person stops walking right in front of the elevator. Her brain has stopped figuring out if the person is a threat or not: the person is not seen as a threat.
13- She stands in front of where the elevator buttons are. Talking to someone. (1:09) Her hands let go, as if now more relaxed. She seems to be explaining her situation ("Sorry I was startled, the elevator isn't working, lol..."), and she relaxes further and now puts her hands on her head. If she was still thinking about dealing with the elevator, she would not have her back to it, she would have called the other elevator. If she was still afraid, she would not put her hands in such a compromising position, especially not in front of someone she felt threatened by.
14- (1:30) The person probably doesn't seem to be able to help her, this person might in fact not be in need of the elevator and on his/her way . She is done talking, the person has moved away from her.
15- She goes back in the elevator. She even looks to be smiling a bit, sort of a desperate "I'm so fed up with this" self-humor kind of smile. An accumulation of problems she has had to deal with in this hotel? She also seems tired or having difficulty to see as she touches the sides of the elevator as she goes in, maybe just exhausted from the drama, or slightly drunk.
16- (1:45) Now she pushes numerous buttons this time, with both hands. This is why I said originally this might not be the first time. See point 2. It explains why she pushed numerous buttons on her first time in the elevator. She knew ahead of time when she first entered that the elevator would likely not budge. Now she knows again, and is desperately trying to make it go down, so she pushes even more buttons. Nothing works.
17- (1:52) The person has moved back. How do I know? Because when she steps out, she is again looking to her left and towards the wall or center of the hall, where someone would be standing, and she has her back to the wall, which implies someone else is occupying space in front of her. It would not make sense for her to stand so close to the wall, with her back to the elevator buttons, if no one was there at all ahead of her.
18- (1:59) She is laughing/smiling as she explains to the person that the elevator is not working and waves her hands in front of the door, feeling silly about the situation.
19- (2:12) Now she enumerates something. Issues she has been having in this hotel? List of items that were stolen from her? Things she has tried to do to deal with her problems?
20- The person tells her to come with him/her. The end.
So my guess is she has been having problems in the hotel, either only with the elevator itself or even beforehand, and she has complained about it or is about to go complain about it. Someone shows up, most likely hotel staff who came up through the stairs. Why? Because someone coming from their room would have headed to the elevator, yet she always stands in front of the elevator button as if the person in question is never even trying to use it, nor does the person ever step in, and the person walks away at some point only to come back. My guess is a security guard, because cleaning staff move with equipment using the elevator. If it was a guest, then it means the guest was going to their room and had come up the stairs, because the elevators are not moving. This seems unlikely, since the elevators were not reported to have been broken, and starts to move as soon as the scene ends.
Who knows what happens next. Possibly the person takes her into the stairway since the elevator doesn't work. If she disappeared at that point, she may have been killed at that point, or brought somewhere before being killed. My guess is she was killed in the staircase, which would cause a lot of noise if she fought, as it would reverberate, hence the noise the old man heard.
Also, note what you can see from the mirror in the picture above: a door leading to the stairs, from where the person would have come.