Just the opposite, actually.

I was hiking in Jasper NP, Alberta this past summer, and on the way up a long day hike, I encountered a guy who was frantically going up the trail asking everyone coming down if they'd found a Canon L-Series 24-70 lens (which is very expensive). He said that he'd left it at the trailhead, and when he realized his mistake, he rushed back down, saw that it was gone, and rushed back up in the hope that someone had picked it up on the way up. On top of this, it was actually his wife's lens that she'd just bought the week before specifically for that trip, so you can imagine the hot water he was in. I told the guy I'd help him find the lens, and when the trail split, I went one way and he went the other. I asked everyone I met on the way up, but people had no lens, only sympathy. I got to the top and the guy wasn't there, so I assumed he'd either found it or given up. It was late in the day and it looked like I was the only person up there, so I put it out of my mind. But about 30 minutes into my hike across the bald, I passed another guy. I was just about to let him pass, but I decided just for the sake of completeness to ask him too, and lo and behold he had the lens! I told him the lens owner's story and got his contact information in case the owner wanted to give him a reward. I put the lens in my bag and hiked on up to the top to photograph the sunset. When I came back down it was about 10PM, and by the time I got to Jasper it was closer to 11, but I sent the lens guy an email, he responded immediately, we met, I gave him the lens, he thanked me, and that was that!

So no, I've never had anything stolen on the trail, but I have witnessed first-hand two honest people going out of their way to return something to its rightful owner.

You can just see the guy I met on the bald here (he's standing on the left crest of the hill), and here's a shot of that day's sunset.