Don't know if all would agree that this is a bug, but here it is.
All hidden items in the game can be picked up by mortals with true_sight.
This could allow for 'hidden' items to be in their inv or on their person/used as a weapon.
Maybe modify do_get to force them to 'search' for the item or if they get the item, auto remove the hidden flag?
Also, keep in mind that Immortals will need to get hidden items from time to time.