I have a bad habit of dropping at least one nut every time I fiddle with the exhaust...and it usually ends up there...

 

I did the same thing while taking off the rear exhaust. nut came out of the socket and I saw it disappear above the primary. I thought the same thing. why the hell is there a hole there? I know it didn't hit the ground. I spent a couple hours using a light and magnet but still no luck. Ran out of sunlight (no garage ) so I had to wrap it up for the day. Will keep looking tomorrow.

Even if I find them, I think I'll take the advice about the ARB nuts on another thread.

 

Compressed air may knock it loose. And I would buy a replacement from HD, not home depot. And yep, I have dropped the rear nuts too, compressed air helped me find them.