You may have noticed from his pictures, that Satchmo has acquired a collar since arriving in Australia. This is because if he goes outside (and we are not 100% sure we can prevent him), the council insist that they be able to tell immediately that he is owned by someone, and that we have paid them the necessary dues to have an indoor-only cat that makes not one iota of difference to their operations.
Satchmo is neutered; he is also microchipped--I am a very responsible pet owner (anyone who says they cannot afford to neuter cannot afford to own the animal in question--neutering is cheap compared to most other veterinary care). If he were outside, and if he were misbehaving, it would be very straightforward to tell he belonged to someone, and who that someone was.
Nonetheless, Satchmo has a red collar that will identify him to council staff as 'owned' (I can tell you how he would feel about the idea that we own him, and it isn't complimentary, I can assure you). Realistically, though, to be sure Satchmo is 'owned', in the event he was seen without a collar, the council would have to trap him and scan him -- because like all responsible pet owners, his collar will break away if he gets it caught on anything, which given he loves to climb is fairly likely.
No matter how you look at it the collar is dangerous, unlikely to be useful, and potentially cruel.