So you had the cage on the floor last night and a bucket trap set and nothing in either? And you heard no noises last night? Did you manage to keep the cats confined to one room last night? If so you could confine them to a different room tonight and set the bucket trap today/tonight in the room where the cats were last night. And put the cage on the floor in that room as well. The idea being have the cats in one room overnight with the door closed and all the other doors open overnight.
As Beryl mentioned I have also heard of people in apartment blocks finding a hamster that had somehow got to their apartment from someone else's. In one case it was via heating vent ducts. In another case it was suspected he got through and adjourning external balcony.
People who find hamsters often put notices up in apartment blocks to say they have found one. You could maybe put notices up saying lost hamster, and a contact number in case anyone finds him.
As Beryl says there are often case of people finding a hamster and taking them in and never finding out where the hamster came from.