Ok this has happened every time playing a 2p split screen game in private crossfire if 1 person picks a sniper and goes into 1st per zoom they become invisible on the other persons screen except for their hand... I dont know if this works during public matches while having guests play but it certainly is a nasty problem which should be dealt with I thought assassins were invisible enough..
Since I've never seen this in a public game I would assume that this is a glitch that only applies in very specific circumstances. Most likely the sniper is only invisible to the guest, or maybe it only happens if you have host. How thoroughly have you tested this?