I agree with Snarf, indoor cats may well be frightened if they find themselves in the big bad world so even more important that they can be identified and returned than a regular 'streetwise' cat. I do confess that only one of mine wears a collar though, mainly because the other one takes on average 10 minutes to get it off so it's pointless putting one on him! On the collar tags instead of 'indoor cat' so the person knows they had escaped, I have 'new to area, if you are reading this he is lost' as I'm aware that some people don't approve of indoor cats and therefore might think twice about returning him/her thinking that they are 'rescuing' them - probably a bit paranoid but there you go!