Odds are slim...
Easy to do, and a bummer. I would say odds are slim that you'll get it back. Did you try the toll shack on Sand Flats? Or the Bike Patrol office at the intersection of Sand Flats and Millcreek (by the cemetery)?
I left my helmet, Camelbak and gloves in the lot at Slickrock last year. I realized it a few hours later, and went back up. Imagine my surprise when I pulled up and here is a girl wearing my helmet, and a guy wearing my gloves and Camelbak, about to head out!
All three were gross and stinky from my ride. I would never have gone near them if they weren't mine! Hell, if I had found them in the lot I would have called in a Hazmat team.
I got them back.