Unless you are planning on resetting your adventures it is often much safer to leave items in stores (yes, you have to buy them back) than on mules in the inn. Think Saz noted that if an item is equipped on a "mule" it is less likely to disappear.
Did you actually save the game with the mule outside the town? Do you have a copy of a saved game where the mule still has the item in question ? I can probably put the item back for you.
Wyv