Dye pack

A dye pack is a radio-controlled incendiary device used by some banks to foil a bank robbery by causing stolen cash to be permanently marked with dye shortly after a robbery.

In most cases, a dye pack is placed in a hollowed-out space within a stack of banknotes, usually $10 or $20 bills. This stack of bills looks and feels similar to a real one, with technology allowing for the manufacturing of flexible dye packs which are difficult to detect by handling the stack.