Glad you found it!
Reminds me of removing my muffler earlier this season and hearing a rattle in there. I shook it out and found a small snap ring fell out of it. I put it on my bench to keep thinking where it would've came from. I feared the engine but it was the standard snap ring with ears, not like the piston circlips so I was stumped.
The exhaust outlet was pinched partially shut and I loaned an exhaust pipe expander from local store and it's threads were all buggered up and it didn't work well so I returned it. I had only tried with the muffler on the sled at that time. A month or so later I decided to give it another try with a new expander tool and removed the muffler and found that circlip.
Long story short, it was off of the 1st tool I used. They use a circlip to keep the tool from being unthreaded all the way. That one had me awake a few nights. LOL