It's pretty near Moab, UT - which is an adult kid's playground. The area is abundant with trails. Much of that land is regularly explored with various 4x4s for sport, even (and especially) off the trails. And Moab would also be a place enriched for people with the skill and desire to make a giant metal slab and find a place to stash it.

The 'monolith' is just a bit SE of the "Dead Horse Point" (SW of Moab), just on the east side of the river on this map [1].

[1] https://www.discovermoab.com/4-wheeling/