Found a few floating rocks, or rather the viking version of the Flying Dutchman, floating ship graves.
Location where the screen shoot where taken
PC
Windows 10
I7-7700K
GTX 1080 TI (latest WHQL drivers)
Yep, seen that as well… like a floating Stonehenge… 
Looks like they modified the terrain mesh at some point removing a hill that was there, but forgot to remove the rocks that were resting on said hill.
Similar to this, I found a room in the Uttern Command Bunker where there are a bunch of floating phones without a table for them to rest on.
I’ve seen books, gas cans, grass, parts of houses/hedges floating mid-air. Also, double graphics on some walls - fire hydrant on top of a framed picture, electric cables running on top of pictures.