Here is another example:

-Full inventory
-M4 with 30 round mag
-Swapping 30 round mag with 10 round mag
-30 round mag disappears due to the lack of space in your inventory instead of being placed in the vicinity

Putting a map together (with full inventory) is another example

This also happens with the ladders of the ATC towers (top ladder to the roof).
I think it has something to do with your weapon stance: if your weapon is not raised but still in your hands, it is most likely that this bug will occur.