Anti-masking
A detector function that reports when someone covers or blinds it so it can no longer see. In Spain the law requires it to be tested on Grade 3 movement detectors at every annual service.
Masking is the simplest attack on a movement detector. Tape a sheet over it, spray it with lacquer or shield it while the system is unset, and when the system is set the sensor sees nothing. Anti-masking is the function that notices and reports it.
The clearest operational rule we have found in an open source is Spanish. Annex II of Orden INT/316/2011, which lists the checks for the annual on-site service, tells the engineer to mask Grade 3 movement detectors, check how they react and confirm the event reaches the alarm receiving centre. On our reading, if the law requires the test on those detectors, at that grade the function is expected. The annex stays in force until an EN or UNE standard on maintenance takes over.
In the UK the product requirements sit in BS EN 50131, which is paid for. We have not read them.
On site the hard part is management. A detector that reports a mask every time stock is stacked in front of it ends up being omitted by the user. Explain to the customer what triggers the signal and where not to put things, and record each event in the maintenance log with its cause.
It is often confused with tamper. Tamper protects the device against being opened or pulled off the wall; anti-masking protects its ability to detect while the housing is intact.
At commissioning, simulate masking properly, with the kind of material an intruder would use. A ticked box in the software proves nothing.