Last time I was at Six Flags (7 years ago, ewww) they had nets under some points on the coasters. We saw glasses, wallets, flip flops, phones, you name it. And it didn't look like they ever cleaned them out and returned property to their owners.
In other notes, just a PSA on all rides to make sure your personal belongings are latched down tight. During my senior trip to Disney World, I lost my wallet without realizing it. The next morning Disney had found it when they cleaned out Rocking Roller Coaster. How it got out of my pack pocket underneath my butt without me even realizing,I do not know! This time I will only wear buttonable pocket shorts for sure!