I've been having this problem, the only thing is, I only have one monitor. It will randomly alt tab out on it's own. And I can't get back in without ctrl+alt+del, and restarting. If I click anywhere I can hear my gun firing but I'm still on my desktop. Sometimes I play for 30 mins and it happens, sometimes only for ten minutes. It doesn't matter if I'm in my inventory, in the woods, or in a city.

Woops, my bad.

This has happened to me from time to time, It seems to do it when swapping items that take the same slot. It seems to happen when there is some server/client lag going on.