An activity gate is a minimum-magnitude test applied to a behavioural feature vector, below which the window is declared quiet and no signature matching is attempted.
Cosine similarity compares direction irrespective of length, which is exactly what a classifier of hazard type needs - and it has a structural weakness: a near-zero vector still has a direction, and that direction is dominated by measurement noise. Without a gate, a quiet room matches something.
The Euclidean length of the feature vector, excluding its duration term, is tested against a configured floor. Below it, no match is reported at all rather than a low-confidence one.
An idle line at night produces tiny movements in every channel. The gate declares the window quiet, and the matcher stays silent instead of naming a hazard class from noise.
The activity floor and the recognition threshold are configurable per deployment; the qualified working configuration uses 0.25 and 0.82 respectively.
