What probably happened is this:
1. BackDoorDisney admitted he's a CM. CM's can get backstage to EPCOT because that's where the Center for Living Well is; aka Disneys private doctor place.
https://www.myclw.com
2. BDD Tells the guard he's there for CFLW, and they let you through. He parks in the backstage parking lot but does his exploration instead (hence why all his exploring is at Epcot, WoL, Communicore, Imagination).
3. BDD is a nerdy kid like 20 years old. He's not the smartest of kin and loves attention. So they post these exploration videos online.
4. Buzzy is stolen, either by BDD, his friends, or construction workers working literally next door via watching BDD's video of him walking around and his path so they know what to do.
5. Disney fires BDD and arrests him, mostly because he's shown leaving fingerprints all over the dang place and the first thing Disney does during on-boarding is....finger print you.
6. At the same time, there is rumors of stuff from Imageworks going missing.
So, BDD may not be THE culprit, but he is absolutely responsible for alot of the missing stuff, if not the stuff from image works.
Backstage videos are cool and all, but it's more and more a bad idea in the modern era.