Device for detecting the opening of a fuel tank cap

The device has a radio beacon (2) and a base station (3), where the radio beacon is mounted inside the tank (1). The detection of the opening of the tank cap (4) takes place by the interruption of a shielding between the radio beacon and the base station comprising a tank wall and the tank cap and the accompanying reduction in path loss that is detected by the base station by a unit (5) for measuring the received power. A grid-like metallic structure is additionally incorporated in the tank to increase the shielding effect of the unopened tanks.