Flying monsters can be in one of three places when the action to move them may occur:
If it is in a location or the streets, the monster moves to a connected street location if there is an investigator there. If there is no investigator, it goes to the sky
If it is in the sky, it moves to any street location which contains an investigator. If there are no investigators in any street locations, it does not move. When there are multiple investigators, it goes to the investigator with the lowest current sneak value. If there is still a tie, the first player decides where it goes.
The below is a quote from version 1.7 of the FAQ. The latest version (2.0) can be found here (it's approx. 25mb PDF.):
- Flying monsters only move when their symbol comes up in the Mythos Phase, just like other monsters.
- Flying monsters will not leave a space that already contains an investigator.
- When a flying monster in a street area or location moves, it will either move to an adjacent street space that has an investigator in
it, or failing that, move to the Sky.
- When a flying monster in the Sky moves, it will move to any street space on the board with an investigator in it. If there’s more than
one investigator to choose from, the monster moves into the space of
the investigator with the lowest Sneak value. If the investigators’
Sneak values are the same, the first player chooses which investigator
the monster moves toward, as usual. If there are no investigators in a
street space, the monster remains in the Sky.