Urine in the food bowl.

About a week ago I saw a cat go under my shed. I was too far away to get a good look at it but it did seem thin so I put some tuna fish out for him that night. Of course, the food was gone by morning. I bought some cat food and gave him some 2 days later. He ate that too. Yesterday he was sitting by the side of the shed looking toward my house with a "Hey human, where's my food?" kind of look about him so I fed him again. This morning there was urine in the bowl. He didn't just spray it. He emptied his bladder in it. I'm hoping this was just a territorial behavior. What do you think?

Sounds like territorial behavior to me.  He wants everyone to know this is his!

If the bowl is ceramic, wash it thoroughly with hot soap and water.  Toss it, if its plastic.  If he persists, make sure you have a supply of plastic or paper bowls.

I would also get him fixed ASAP.

It's a small stainless steel bowl and it's soaking now. Paper bowls sounds good.

If he stays I could borrow the trap my friend used to trap and fix her feral cats.

I don't think he's gonna be a feral for long acting like that. It's a strong deceleration of ownership.. he liked it and you so much he put a ring on it... Lol