Intruder alert would benefit from not needing much precision. A lifeformish thing suddenly appears -> intruder alert, regardless of whether the exact or even coarse nature of the thing can be established. A door is forced -> intruder alert, regardless of whether a Klingon, an utterly invisible creature, or nobody enters. One doesn't even need a process of elimination, such as footsteps heard -> is anybody legit present, as per tracking records? -> yes -> no alert. False positives are not a big problem, and false negatives in cases such as the above are covered by other things such as the legit person indeed being there to witness.