Sudden lethargy & missing collar

My 8 year old indoor/outdoor neutered male has a history of cat fights and abscesses. We usually don't know he's fought until we feel puncture wounds or see an abscess. (Please no lectures on keeping him indoors. He had been living outdoors for two years when we adopted him from our neighbors and we've been unable to turn him into an indoor cat). Today I came home from an errand to find him curled up on the couch (unusual for the time of day), with his collar missing, and very lethargic. He let me pet him, pick him up, and feel him and didn't wince or appear to be in pain, and I felt no punctures. An outside noise scared him and he went under the bed, which is where he goes when scared. He won't come out or eat or drink, though he di

How is he doing today? Lethargy is quite an extreme condition. If he went under the bed maybe he wasn't entirely lethargic. Lethargic means apathy and lack of response to stimuli. He may just have been feeling very weak and tired (which is concerning, definitely). There are so many things that can happen to a cat to cause this. Anything from poison ingestion, being hit by a car, an accident while fall/jumping from a high place (landing on those feet doesn't always work). It could even be a bug. It could be an infected wound too, as you probably know, the infection and fever can make a cat very quiet and not prone to moving but then we would expect to see a reaction when you felt him over and that didn't happen.

Not having the collar may or may not be related. Assuming it's a breakaway collar, he may have lost it in an unrelated incident?

How is he today? Has he been eating/drinking, using the litterbox? If he's still unwell, I definitely would talk to the vet. 

Thanks for the response.  It turns out it was a fight after all.  I didn't see any wounds when I first posted the message, but a few hours later I found several.  I ended up taking him to the vet the next day.  They gave him a shot of antibiotics and anti-inflammatories for a few days.  I was worried because I had to go out of town the next day, but the house sitter said he was fine.  I just returned home today and his wounds are healing nicely.  No fluid underneath them or anything.  He's returned to his playful, hungry self!

Good news!!