OK First time i Thought i'd just dropped my mag when drinking at a pump.
What happens is, if you have more than one magazine, and you are holding your rifle when drinking. The scroll option comes up as "Drink" "Change Magazine" Change magazine"
so if you accidentally select "change magazine", the currently equipped mag will dissapear from your inventory. My inv has been almost full both times, not sure if that makes a difference. It has happened with a 30 round m4 mag and a 60 round dual clip mag, so far.