Caltrops, Returning (3.5e Equipment)
From D&D Wiki
The Returning Caltrops are a creation of the alliance between the gnomes and dwarves of the Second Age, during this time their peoples were being relocated and forced to live with one another as slaved to the growing Sarosian armies. The dwarven skill at crafting and the gnomish talent for magic and tinkering combined to create many powerful tools and weapons.
The bag of Returning Caltrops follow the same rules as regular Caltrops regarding placement, damage and effects. The holder of the bag of Returning Caltrops may speak a command word if still within 240' of the 5' square they placed the caltrops, if they do the caltrops will dematerialize then reappear in the bag.