Glad he is now chipped.
Right what i would do -
If he comes back with another collar on attach your phone number to it and see what happens.
It could be that he has been "faking" being an un-owned cat !!
I had this last week with a ginger cat that turned up at mine, he wouldnt go away and always seemed hungry. I listed him as found at all the local vets. I got him neutered (as he wasnt done) and then he went missing.
He returned a couple of weeks later with a collar on so i put my phone number on it.
A couple of days later i got a phone call from his owner
We had a long chat and i said what i had done, the owner wasnt impressed that i had had him neutered but thats tough tittys !!
The cat still pops round every now and then - although i havent seen him for a while now