This sort of thing is why I've stopped posting on nextdoor when I trap a Cat. I just keep quiet about it and look for shelter placement, if it turns out there's a microchip then I'd let them handle reuniting Cat with person.An update, but not a happy ending.
I posted to Pawboost, purchased a Facebook ad (through Pawboost), and paid a little extra to make him a Featured Pet on Pawboost. Then I posted to Nextdoor. His "owner" saw the Nextdoor and contacted me. It does not sound like an ideal fit. Myself and the rescue pleaded our best case for rehoming. But she insists she wants "her" cat back and there's little we or the rescue can do about that. The "owner" is a neighbor who says she "rescued" him from the streets (our community) last summer, neutered and chipped him. She also insists that he won't be contained inside. I don't want to get into a neighbor feud. So I guess I will be seeing him around again. I asked his "owner" to put a collar on him but we'll see how far that goes. She was not particularly pleasant or compassionate in our interactions.