This is really mostly a warning to everyone that actually uses boxes and leather bags to manage their inventory.

Edgin died and when he respawned, a very nice mithral shirt was missing from one of this boxes. I've noticed that things resort in your boxes when you die, and if through the resorting an item that used to fit no longer does, it disappears.

Be wary of what you put in boxes and bags... you'll actually want to leave plenty of room in them especially if you put things of varied sizes in them.

You're sure someone didn't take it? :)

It's a pretty old bug related with, as Macizo says, the way the game automatically sorts equipment, which is a way that is less than optimal, combined with the fact that items that do not "fit" after the sorting simply disappear. That's why the common advice is to only stack items of the same inventory size in a particular bag or box, or just leave ample room for rearrangements. Sucks, but we've got to deal with it, I'm afraid.

Yup... I confirmed with the DM that no one took it. Though that is how it's being played IC.

Just a box/sorting issue I'm afraid.

Don't fill boxes personally with any more items than full horizontally, else you'll lose something people. Specially if it's expensive.

The easiest way to handle it is to only put items of the same shape in a box or bag. If you fill it with 2x2 items for example, no matter what order it sorts in, they will all fit. The problems come from mixing large items with small items, and then there not being room for the large item if it goes in towards the end.