IIN I keep finding this cat at night?

The lack of collar means nothing.

I've shared my home with several cats over the years, and I stopped putting collars on them after seeing how one reacted when it was squeezing through somewhere and the collar got snagged. Rather than gently backing out, it panicked when it felt something grip tight around its neck, thrashed around and got itself even more entangled. Cats aren't bright enough to figure out how to deal with a snagged collar, and they regularly poke around in tight places and go up trees which have lots of snag-hazards.

This is why good cat collars always have a section that's elastic. That supposedly allows enough give for the cat to extract itself if it gets snared, but I've never trusted that theory.

In any case, I've never felt a need to put a marker on our cats showing that they're owned, mainly because I've never really believed that I do actually own them. They've all seemed happy enough to hang out with me, but I know that's only because I've provided a warm, safe environment, and a regular supply of food and attention.