Indication device for indicating unauthorized opening of a door of an aircraft parked on the ground, having a sensor for capturing a distance between the sensor and an object in a capture range in front of the sensor, a signalling device which has a first state and a second state, a reset device which is coupled to the signalling device and is set up to change the signalling device to the first state if a predetermined reset operation is carried out by a user, an activation device which is set up to selectively provide a predetermined deactivation signal and a predetermined first activation signal as an output signal, and a control device which is coupled to the sensor, to the signalling device and to the activation device and is set up - to change to a first operating mode if the deactivation signal is received, and - to change to a second operating mode if the first activation signal is received, in which second operating mode the control device detects a change in the distance between the sensor and the object, on the basis of a sensor signal which is provided by the sensor and is characteristic of the distance between the sensor and the object, and changes the signalling device to the second state if a change is detected until the predetermined reset operation is carried out, with the result that, after the sensor has been arranged on a door of an aircraft at a distance from a stationary component in the interior of the aircraft or on the stationary component in such a manner that the stationary component or the door is in the capture range of the sensor and opening of the door changes the distance between the sensor and the stationary component or the door, and the first activation signal has subsequently been provided by the activation device when the door is closed, a change of the signalling device from the first state to the second state indicates that the door has been opened, wherein the indication device is in the form of a cohesive compact structural unit which can be integrated in a door panel of an aircraft door, wherein the indication device has an optical signal device for emitting an optical signal, which optical signal device is activated when or after the signalling device is/has been changed to the second state, wherein the indication device has an acoustic signal device for emitting an acoustic signal, which acoustic signal device is activated when or after the signalling device is/has been changed to the second state, and wherein the optical signal device and the acoustic signal device are part of the structural unit, characterized in that the control device is set up to operate the optical signal device in the event of an approach of a person, as captured using the sensor, to within a first limit distance and to operate the acoustic signal device in the event of an approach of a person, as captured using the sensor, to within a second limit distance, wherein the first limit distance is greater than the second limit distance. - to change to a first operating mode if the deactivation signal is received, and - to change to a second operating mode if the first activation signal is received, in which second operating mode the control device detects a change in the distance between the sensor and the object, on the basis of a sensor signal which is provided by the sensor and is characteristic of the distance between the sensor and the object, and changes the signalling device to the second state if a change is detected until the predetermined reset operation is carried out,