A method and a device for detecting intrusion into or tampering with contents of an enclosure. According to the method at least one gas proof seal-barrier is provided in association with the enclosure and a first environment is produced within said gas proof seal-barrier. Said first environment is different regarding at least one characteristic feature from a second environment surrounding said gas proof seal-barrier. At least one detector is arranged within said gas proof seal-barrier for detecting a predetermined change of the characteristic feature of the said first environment. Said detector is transferred irrevocably from a first state to a second state when there is a predetermined change of the characteristic feature, and the state of the detector is supplied to a position outside said gas proof seal-barrier.