Sketch of the retrofit
Most of the time, the screen just relays the video of the scene, and the security guard or video analytics system is unaware of the intervention (in most surveillance video quality degradation would be hard to notice). However, according to the software running inside the Retrofit's processor, the video can be subtly altered, for instance blurring the faces slightly, or even altering them (as proposed by Newton et al.). More aggresively, parts of the scene could be blanked out, perhaps by superimposing an image of a parked lorry, or an umbrella over every person. The retrofit could crudely blank out faces, show a view onto a virtual world, send messages to the control room, or just transmit cable TV programs - the security guards might be entertained enough not to investigate what was wrong with their system.