Ask around to see if any neighbors have outdoor cats, have seen them, or may know where they came from.

If any are approachable enough to catch, you could bring them to the vet or shelter to be scanned for a microchip.

Otherwise... I think it's safe to assume they're strays and they figured out your house is a safe haven for stray kitties.

Be cautious though, as if you're feeding strays you're legally responsible for their care, so if one gets sick or gets into a fight and is injured and it goes untreated, you could get in trouble Also be cautious not to get bit or scratched (I'm sure that goes without saying though). I know you're probably aware of all of this though!