Belts can slip off too. The op's story, which is not unique, points out the importance of a pre-dive buddy check which should include the security of the weights however they are carried. I was on a dive in Cozumel when a member of the group lost a pocket at about 90 feet. I saw it drop, retrieved it and returned it to him. His buoyancy was minimally affected at that depth. The effect is more pronounced the shallower you are. I also have seen students lose an entire weight belt, bcause they do not wear it at their waist. Your people in particular seem to not know where their waist is.
DivemasterDennis
DivemasterDennis