Look for “key terrain” – entrances, exits, paths to and from entrances and exits, cover and concealment, etc. Do you have clear fields of view (and fire)?
Look for essential elements of friendly and enemy information – Who’s hanging around? Who’s a likely potential threat? Is somebody acting in an odd or threatening way? Who or what is out of place? Long trench coat in 100deg heat? Somebody muttering to himself or a nonexistent person?
Common sense works as well at the ATM as it does in the Korean DMZ.