How do you know how to find a human when the announcement comes in that they're about to be freed?
And what triggers that? Can I do anything to speed up their release?
When you hear, 'Keepers detected', a special group of glowing green enemies spawn. Some spawn in a certain area, and follow a short movement pattern before disappearing off the map (thus, losing that human if you don't kill them in time), some spawn and stay and patrol the map. There is occasionally also groups where you must kill the group in a specific order, if you kill the one that isn't glowing green first, you automatically lose that human. Once you kill them all, a human is freed.
You can tell which one is going to be freed ahead of time because the box with the human will be flickering letting you know once you kill those enemies, this will be the guy that is freed. A lot of the time though, you won't be around there to see that, so you need to rely on the arrow after the fact.
Once you kill the keepers and the human is freed, an arrow appears I believe around your fighter letting you know which direction to go to find the stranded human.