I mean, if its found at a dump or something? As long as you weren't trespassing, I don't see what the problem would be. I've got a bolt from AstroWorld's Serial Thriller, when it was sitting in Great Escapes parking lot for a few years, the whole ride was just sitting in the parking lot, not even fenced off or anything and the track and parts were just.... there. Found a bolt just sitting in the dirt and kept it
(and they probably replaced alllllllll those bolts when the ride moved to LaRonde annyyyyways)