Its probably in the belt ribs. Done the exact same thing once,really hard to see.good luck!

 

+1 on getting a magnet.

 

I'd check the garage floor. Amazing how far those little suckers can roll. I spent an hour one time searching my bike just like you, only to find it down at the curb at the end of my (sloped) driveway the next day when I was doing the yardwork.