An object of the present invention is to provide a mechanism for tamper detection of electronic devices (110) in closed units which is robust and low cost. The object is achieved by a method in an electronic device (110) for detecting if a cover (100) enclosing the electronic device (110) has been opened. The cover (100) comprises an enclosing assembly (250) which is adapted to fasten the cover (100) into a closed position. The electronic (device 110) comprises a non volatile memory (120). The non volatile memory (120) comprises a stored reference signature associated to the enclosing assembly (250) when the cover (100) was fastened into a closed position. The method comprises the following steps: (Creating 1003) a signature associated to the enclosing assembly (250). Comparing (1004) the created signature with the reference signature. Detecting (1007) that the cover 100 has been opened when the comparing (1004) results in a difference.