Abstract: A device for detecting and signalling or indicating status as regards contents in a container, and in particular a letterbox, comprising a detector for registering a change of state in the letterbox, which registration is transmitted to and activates a preferably remote display or the like in order to produce on the display a visual and/or audible message which signals that contents have been deposited in said letterbox. The detector is arranged in a preferably integrated mounting unit made in the form of an approximately U-shaped holder that accommodates a circuit board containing electronic components, a light transmitter, a light receiver, and wires and power supply, which unit is placed on the bottom of a container, preferably a letterbox, in such manner that the two upright legs of said holder rest against respective opposite walls of said letterbox. A detector and transmitter unit and a receiver-display unit are also described.