As a few people have said already. It mainly happens to me and my group after we have taken cloths off a body, 5 minutes later when we get to the trader we are in our underpants. So I'm guessing it has something to do with getting in and out of a vehicle. But I might add also, it isn't just cloths, it happens to our rigs/vests also. So most of the time when we log back in the next day and no one has said the previous day 'you naked' both our cloths and rigs are gone along with the gear.
Hope this helps!