I have dreams where I have so many cats that I can’t remember their names, and sometimes accidentlly leave them in the back of the closet for years.
The cat could be microchipped - a local vet or animal welfare organization could probably scan it and tell you if an owner’s name is on file.
This thread is useless without pics. 
I had “named” the cat Ninja because of the silent, secretive way it got into my apartment!
For all who asked, even though she wasn’t fat like my Poncho, she was by no means unfed or under-fed. Her fur was not matted/disheveled.
But here’s the thing: I took my daughter to the theater last night in Morgantown (an hour and a half away) and when we came home, Ninja was nowhere to be seen.
My new theory is that there must be a hole in my basement somewhere that’s large enough to let a cat in/out. It is not unusual for me to leave my basement door open, as my laundry room is down there.
I will have my handyman check over the next couple of days. It’s getting cold 'round these parts, and I don’t have to be paying to heat the neighborhood! 
I had a similar thing happen about a year ago. My dial tone on the phone stopped working. So at 8am the phone guy shows up. I live in a block of flats, so I showed the guy to the back where the phone room is, and I didn’t close my door all the way.
So I get back, and watch TV, and about 8:20am the phone guy has it all fixed and leaves. About 9am I’m sitting on my bed and a cat jumps out from under the bed and sits on my legs.
It’s scared the life out of me. I was like “How’d that cat get in here.” He/she was very affectionate, and I reckon she came in when I showed the phone guy where the phone room was.
I just put her back into the hall, I reckon she belonged to someone in the building and just snuck out the door when they weren’t looking. I never saw that cat again.