SweetCat,

If it is safe for him outside (should be safe as you are in an apt), then you wouldn't need to bring him in lor.... unless if he is showing signs of distress / wounds or needs "warmth" or medical attention ...

Since you have posted notice around your apt, I think all you can do is wait for someone to call...

BTW, where does he do his poo and pee? The owner may be on the same floor but some cat owners, you knowlah, too busy, let their cat roam around for food... I have another suggestion, maybe you should drop notices about the CAT in mailboxes of all units on the same floor (not sure how many would that be!). Remember not to give a photo, but descriptions, when they come forward, test them to make sure they are the owners!

If all fails, with no respond .... keep him? Otherwise consider finding him a good home! (wouldn't want him to eat KFC bones for the rest of his life!).....