I've done a bit of testing with this. Changing locations doesn't cause the problem, i've gone in and out of different buildings about a hundred times to push an area transition and that doesn't cause the problem - no items lost.
I've sold and rebought gear, equipped different stuff to different people, entered into and out of combat, completed quests and levelled up multiple times - no items lost.
When I died, and it had to reload my save game, that caused an item lost. Tested multiple times from new games, lost combat, reload game - items are lost, randomly but it seems to target weapons the first time in the 10 times I tested this. Multiple successive kills/reloads causes further items to disappear, moving from weapons onto armor. No inventory items so far but that could be chance.