A Passive infra-red detector sends a signal when a target which matches its porgrammed respone moves in its field of view. But the Passive-Aggressive detector is more sophisticated; it notices the target, but doesn't do anything immediately. Instead, it pretends to be busy; it whistles quietly to itself, hums, or looks the other way. No lights, no sirens, just vague, irritating noises.

Not unexpectedly, if the target is human, they will be intrigued by all this, and like as not will go and have a look at the detector, which continues to studiously ignore them. Eventually they give it up as a bad job, and move on.

After a bit more humming and clicking, the Passive-Aggressive detector triggers the Claymore mine hidden just round the corner, nicely timed for when the target is passing by it.