An alarm kit comprising: an elongated sensor assembly for detecting a change of state comprising: - at least one magnetically activated sensor switch, such as a magnetically activated Reed switch, configured to detect a given state and a change of state between the given state and at least one other state, - a microprocessor configured to detect the change of state of said at least one sensor switch, - an antenna system, - a wireless transmitter configured to receive a signal from the microprocessor identifying a change of the state of the at least one sensor switch and transmit said signal by means of the antenna system, and - a power source for providing electric power to the microprocessor, the at least one sensor switch, and the wireless transmitter, wherein said at least one sensor switch, said microprocessor, said antenna system, said wireless transmitter, and said power source are incorporated in said elongated sensor assembly to reach a maximum height of the sensor assembly of less than 5 mm, and at least one actuation unit for actuating the sensor switch, wherein the at least one actuation unit comprises a magnet, wherein the elongated sensor assembly and the actuation unit are configured laterally displaced to each other in an opening, void or cavity formed by a window or door and a corresponding frame in a closed position of the window or door. an elongated sensor assembly for detecting a change of state comprising: - at least one magnetically activated sensor switch, such as a magnetically activated Reed switch, configured to detect a given state and a change of state between the given state and at least one other state, - a microprocessor configured to detect the change of state of said at least one sensor switch, - an antenna system, - a wireless transmitter configured to receive a signal from the microprocessor identifying a change of the state of the at least one sensor switch and transmit said signal by means of the antenna system, and - a power source for providing electric power to the microprocessor, the at least one sensor switch, and the wireless transmitter, wherein said at least one sensor switch, said microprocessor, said antenna system, said wireless transmitter, and said power source are incorporated in said elongated sensor assembly to reach a maximum height of the sensor assembly of less than 5 mm, and - at least one magnetically activated sensor switch, such as a magnetically activated Reed switch, configured to detect a given state and a change of state between the given state and at least one other state, - a microprocessor configured to detect the change of state of said at least one sensor switch, - an antenna system, - a wireless transmitter configured to receive a signal from the microprocessor identifying a change of the state of the at least one sensor switch and transmit said signal by means of the antenna system, and - a power source for providing electric power to the microprocessor, the at least one sensor switch, and the wireless transmitter, wherein said at least one sensor switch, said microprocessor, said antenna system, said wireless transmitter, and said power source are incorporated in said elongated sensor assembly to reach a maximum height of the sensor assembly of less than 5 mm, and at least one actuation unit for actuating the sensor switch, wherein the at least one actuation unit comprises a magnet, wherein the elongated sensor assembly and the actuation unit are configured laterally displaced to each other in an opening, void or cavity formed by a window or door and a corresponding frame in a closed position of the window or door.