do the cats know they are indoor cats? - can you absolutely guarantee they will never get out? cause if they do and they aren't used to outside, then they are more likely to get disorientated and lost than cats who wander in and out daily.
i think they do. When anyone comes into the flat, they scupper under the sofa bed. Sometimes when I let them wonder around the communal stairs, and they hear a door opening, they run back into the flat in a panic.