Odd wound on cat

Hi, firstly let me note that we HAVE seen the vet and had this wound stitched. I noticed a wound on my cat yesterday in his arm pit. It was hard to get a good look but I thought a burst abscess. Took him in to the vet first thing this morning and when they moved his leg they realised it was actually a very deep tunnel. They think it’s possible/likely that he’s gotten caught on something and ripped himself free. There’s also a smaller wound below the big one. they took him in for emergency surgery and found the tunnel was quite long - half way across his chest and through the fat layer. Narrowly missing nerves and his chest cavity. I trust them and their diagnosis but I’m just so confused because the wound was so clean - no blood etc (although I guess maybe because my cat was cleaning it?) We just have no idea what has happened and I can’t find traces of blood etc inside or outside (he could’ve hurt himself in a neighbours yard though). He's now officially an inside cat, we aren’t going through this again. But I just need to convince myself that this was just a puncture and not some kind of medical condition that’s caused a tunnel. Hes 6yo male, desexed. His vaccines are most likely out of date or incomplete. He had a urinary blockage 2 years ago and is still on urinary s/o food. We are in Australia. One other cat (his biological brother). This picture doesn’t show how deep it actually is

Hello - obviously your vets are in a much better position to understand this wound than I am, having explored it, so it's worth asking them exactly what they think it is.   I wondered about a foreign body - its not uncommon for a grass seed or similar to burrow through the tissues and then come out somewhere else.  They have very sharp ends and can cut surprisingly long tunnels.    Sometimes we find a foreign body on exploring these wounds; other times, what you found was the exit hole rather than the entrance hole, or the body has already thrown an inflammatory response, walling the foreign matter off in an abscess.   There are certainly other explanations and I cant exclude but wounds or cancer, for example.  Good questions for your vet are 'what could it be in my cats' case?' and 'what is most likely,' 'are there tests we can do to know for sure' and 'what is the best course of action from here?'