I found the blue-handled Benchmade Mini-Griptilian 555 (with the modified sheepsfoot blade and oval opening hole) that I thought I'd lost last Thursday.
I took another look at the ratty thrift store sports jacket I'd worn that night. I'd already checked the pockets that I actually use, but it occured to me that the clip might have hitched a ride on the lining.
The thing about knives with pocket clips (or anything where the clip stays on the outside of the pocket), is that they can snag on seat belts, jackets, etc.
This was especially true because my jean pockets that evening were comparatively shallow.
Anyway, it turns out that that ratty thrift store sports jacket has an additional set of pockets on the inside, below the standard interior pockets.
The clip had snagged on one of those.
Now if I can only find my Shuffle.