Actually, after reading my above post, it was a ~10mm clamp. I initially remembered it as a heater hose clamp, but it was for a throttle body hose. Still, lost is lost, but at least it was smaller than something I could fit on my thumb. It's probably on top of one of the trusses that hold up the next floor above the garage.
While not lost, and not a bolt, one time I changed a timing belt on a car the day before a long trip (yes, I know, I'm a genius). One of the timing cover screws wouldn't thread into the weld nut, so I ran a tap thru it. Finished the job, all is well, drive 1000 miles the next day and check the oil when I get gas. When I opened the hood, there's the M6 tap sitting on the top of the fender, that 1" wide area where it bolts to the body, right below where the hood closes. Evidently, when the hood was closed it trapped it there

I'm way more careful about setting things down like that since then.