This has happened numerous times. I place any item on the ground and about half the time it disappears. I leave the area and come back; still gone. I wait a few minutes; still gone.

I spent 2 hours last night getting the nicest M4 I've had. Today I spent 1 second placing it on the ground and now it's gone.

But I guess it's better than my character completely being wiped.

EDIT: I'm walking into a wall to glitch to see through it and I see the M4 I dropped. Currently trying to pick it up through the wall. But I think the problem is that when I dropped it the M4 was actually dropped a few feet in front of where I was causing it to glitch into a wall.

EDIT2: I GOT IT! I kept walking towards the wall to see through it. When I saw it I vaulted into the wall and was able to pick up the M4. Then I was able to vault back into the room with no problems and the retrieved M4.