My Barbarian had a blue item 'drop' from the small group of undead stood outside the Tristram Cathedral (Act1/Inf). It was named 'Ominous Codex'.
When I clicked the item, there was a strange bell-like sound effect that I'd not heard before and the item disappeared as if I'd picked it up. But when I checked my inventory, there was nothing there...
A Codex is a high level variant of the Wizard's offhand item, like Aconite said above. They do make a certain chimey noise if you drop it or place it into an inventory slot.
Not sure what might have happened to the OP's one though. Might have been during last night's lagfest (which seems to kinda, sorta be happening again, right as I'm about to go kill The Butcher with my baby Wizard...)