My poor kitty. Trigger warning

My cat ran out while I was taking my dog pee a few days ago and went missing. I’ve searched the whole neighborhood, knocked on neighbors doors to see if they’ve seen him.. everything. I thought he got ran over or died or something and then today he came home. I was so happy to see him but he looks like this and I can’t stop crying. It looks like it’s healing already at least. I’m taking him to the vet to get checked just in case. There’s no puncture wounds or anything and it he has no other marks at all. it almost looks like a burn but how in the world could he have gotten a burn? Does it look like a bite or burn to you? He’s acting totally fine like nothing happened but he’s getting extra snuggles and treats right now.