I've done these things to find his owner:
Walked to all the apartments nearby (20-ish) and asked if he belonged to them
Took him to the pound to get his microchip checked (but it wasn't registered and came back registered to the pound he was picked up at so they are supposed to call me on Monday and see if they can locate the owners)
Let him out several times to see if he'd just run home
I feel bad just leaving him outside. He has no food/water out there. He's someones cat because he's pretty clean and well behaved. He also has a collar (but no tags).
Where do I go from here? Just keep up what I'm doing until they call Monday (I'm making some flyers right now to post around the complex)? I love him. He's so cuddly and cute and loving. I know someone else loves him and misses him, too.