Weapons disappearing?

Hey, I recently started playing and I've discovered a couple weapons - my first was a CZ-50 or whatever that scoped rifle is. I wanted to continue using my axe for the moment, so I opened my bag to put the rifle in, but as soon as I clicked it, it disappeared and it was nowhere to be found. Not on the floor, not in my bag, nowhere. I then had 4 mags with no gun.

Anyway, I'm just wondering if this is a well known issue, and if it is, what caused it, and how to get around it? Cheers.