A device for monitoring an entry to or exit from a space via an access point, the access point having a first component and a second component that are separable from each other to create an opening for the entry or exit, the device comprising: a first part for mounting to one of first component and the second component, the first part having: a sensing component for sensing in multiple dimensions a magnetic field emanating from a second part mounted on the other of the first component and the second component, and a processing component 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; wherein the processing component is configured to detect a change of condition at the access point, by: a first comparison, with respect to a first threshold, of a first dynamic quality that represents an amount of change of the sensed magnetic field over a first duration of time; a second comparison, with respect to a second threshold, of a second dynamic quality that represents an amount of change of the sensed magnetic field over a second duration of time that is different to the first duration of time; and detecting a change of condition at the access point when at least one of the first value is greater than the first threshold and the second value is greater than the second threshold. a sensing component for sensing in multiple dimensions a magnetic field emanating from a second part mounted on the other of the first component and the second component, and a processing component 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; 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; wherein the processing component is configured to detect a change of condition at the access point, by: a first comparison, with respect to a first threshold, of a first dynamic quality that represents an amount of change of the sensed magnetic field over a first duration of time; a second comparison, with respect to a second threshold, of a second dynamic quality that represents an amount of change of the sensed magnetic field over a second duration of time that is different to the first duration of time; and detecting a change of condition at the access point when at least one of the first value is greater than the first threshold and the second value is greater than the second threshold. a first comparison, with respect to a first threshold, of a first dynamic quality that represents an amount of change of the sensed magnetic field over a first duration of time; a second comparison, with respect to a second threshold, of a second dynamic quality that represents an amount of change of the sensed magnetic field over a second duration of time that is different to the first duration of time; and detecting a change of condition at the access point when at least one of the first value is greater than the first threshold and the second value is greater than the second threshold.