Second or third time this happen to me now. All of a sudden pants disappear when in inventory. But has also happen when logging in to a server, I stand then without pants when I am fully log in to server.
Last time today I moved some stuff from my backpack to a backpack on ground. Not anything from the pants itself. They just disappeared..
EDIT: I am talking about those pants I am wearing on my character. Not stored in backpacks or in other storages on character.