It would not be the first thing
they have made to vanish...
https://www.otherhand.org/home-page...llison-mine-hike-circa-1973-yes-friggin-1973/
That link goes to a story about a very substantial mining operation, or the ruins of one, that mysteriously disappeared in the 70s. It's an interesting story about an interesting place, with video and photos.
The "monolith" (which is really just an odd shaped steel box) could have been carted off by others, but I doubt it. Photos show it was set into a hole formed by three saw cuts in the rock. Those cuts were made by someone with experience and a serious machine, though it was probably a concrete saw with a chainsaw engine, so portable enough. Anyway, I don't think it would have taken much pushing the structure back and forth to loosen it.
What would one do with such a "prize"? Keep it in the secret clubhouse?