A device may monitor an entry to or exit from a space via an access point. The entry access point has a first component and a second component that are separable from each other to create an opening for the entry or exit. The device may comprise a first part for mounting to one of said components and which has a processing component and a sensing component for sensing in multiple dimensions a magnetic field emanating from a second part mounted on the other of the first and second components. The processing component is configured: to receive an indication of the sensed magnetic field; detect a change of condition at the access point when a dynamic quality of the indication of the sensed magnetic field satisfies a predefined criterion; and output an indication of the detected change of condition. There may be a corresponding system, method and memory.