Every item has a unique ID #. Each ID# can only belong to one person at a time, and you can only save items to the database if you are the owner of that ID #. I think ID #s are being reused every once in a while, which would explain the random disappearances! For example, you have an aurelian platemail with ID# 5555. If the server generates a club with ID # 5555 and someone picks it up then drops it, the person who picked up the club will take away your ownership of ID # 5555, then will erase that ID entirely (nobody owns it). As a result, when you log off + on, your platemail won't be saved.
That's probably more detail than necessary, but the jist is that I'm pretty sure I can kill this bug once I get my computer back.