Some days ago, a friend of mine looted his past dead body and I found that his shirt was totally invisible, both of us could see the objects behind his chest, so, maybe this is possible and some players know how to deal with this.

I was thinking that maybe the guy who killed corellor was hacking, is another possibility

before you discount this thread, i would like to add that i suspect there may be a way to render textures invisible in this game. recently, i was attempting to position myself to locate and execute a sniper when a "fresh spawn" entered the same building as i was in. there where a couple very well kitted people dead on the floor near me, so i let him approach and told him he could help himself to the gear they where carrying (i didnt kill them, the sniper did). he went prone near the stairway to the room i was in, nowhere near the bodies lying on the floor then his hands positioned themselves as though he was carrying a mosin. before i could think wtf, i was dead. i heard the shot that killed me, and its audio position matched the "fresh spawn" who had appeared to be carrying no gear with him, only pants shoes and shirt. the sniper that i was hunting was far enough away that their gunshots didnt even make noise i could hear.
so, as i said, i think there is a way to render some textures invisible. if this method where applied to clothing, it would essentially create an invisible character.