She sent an anonymous tip to David while she was staring Jefferson down, right before she flashforwarded to the airplane.
We don't actually see that happen... but I guess I'll just go with that as a way that would actually explain it.
Didn't Max send a text message revealing the Dark Room's location?
Oh wait, is that totally the key element that I'm missing?
...It has to be in the main menu where you see her text messages, right? Should have thought about checking that.
Alright, my issue with that one sequence is solved.
Edit: Actually, brain fart: https://youtu.be/iMp4CgdBmEo?t=27m