Stability
From Arkham Horror Wiki
Each location has a stability rating, indicated by a green (
) or red (
) diamond on the location's area. Green locations are stable: monsters and gates will not appear in these locations, although monsters may still enter through normal movemment. Red locations are unstable: monsters or gates can appear in these locations as a result of an encounter or drawing certain cards (for example, a Mythos card).
Unstable locations, while more risky, also give players the possibility of encountering an Ally, and Clue tokens often appear in unstable locations when Mythos cards are resolved.
