This occurred to me in the low-rise apartments in Chapaevsk (3rd floor) while looting, I dropped the weapon (Mp133 shotgun) from my hands onto the floor - the gun sank partially into the floor, and I could only find a spot to pick it up from with great difficulty.

Dropping a Mosin shortly later produced the same result - the gun mostly disappeared into the floor (only the adjustment knob on the scope was visible). I spent considerable time checking the area carefully - "pick up" menu option did not appear, and the rifle did not appear in the vicinity with inventory open, so I was not able to pick up the rifle.