Abstract: A security device for transporting documents of value such as bank notes, comprising an enclosure having a space for receiving the documents and housing a foam generating unit including a receptacle containing marker liquid adapted to foam up and color documents, said receptacle having openable closure means, a reserve of compressed gas having means for releasing compressed gas, actuating means for controlling said means for releasing compressed gas, at least one movable wall disposed inside said receptacle between said marker liquid and said means for releasing compressed gas, a liquid/air mixing means connected downstream of said receptacle closure means for converting marker liquid into marker foam in response to compressed gas released from said reserve urging said movable wall to force liquid via said closure means to said liquid/air mixing means, and discharge means for carrying marker foam from said liquid/air mixing means to said space for marking said documents.