Hmmm...a key lost in the snow off my back porch, which a week later was just visible as it reflected sunlight.
A nice flashlight I'd been looking for, and almost a year later found under some towels in the guest bathroom.
A Rickenbacker 4001 I'd had to check in on a flight, but it took the airline 1.5 hours to find and return to me at a different luggage retrieval spot.

But the best wasn't so much a lost item, as the kind of story I refer to even to this day when fatigued/bonehead confusion stories are told:
We were building a guy's Southwest Tour car - kind of a tube-framed TransAm car - in a mad thrash to make a race in a couple days.
Working straight through for at least 3 days, an occasional 1-hour rotating nap schedule, so an intense thrash by 3 of us with occasional help.
Stuff is spread out among 1,000 SF, different tasks of course being attacked simultaneously.
We're down to final stuff with a day left, it's the wee hours, we're nearly zombies.
Car owner's looking for the Whitney Punch hand tool, tearing around, swearing as an ex-Marine can, "where's the GD/F'n/etc whitney punch", over & over.
We finally drop everything to frantically help look to calm him down.
It's been in his hand the whole time.
We sent him to the sofa for a nap.