[rarestia]

If several of the others had it, it increases the odds of their have recently consumed it, and thus impairment.

I don't follow. So if you had 10 people in a room and all of them peed in a cup and came up positive for THC, it's reasonable to assume they all toked up recently?

THC binds to fat. The longer you smoke, and the more body fat you retain, the longer detectable THC stays in your system. It's not an accurate indicator of when it was last used, just that it was used sometime in the past.

[yefragetuwrabrumuy]

This is not definitive evidence, but is useful. If say, one of the six did not have any measurable amount in their blood, suggesting that they did not inhale smoke actively or passively within the window of detection, either they were separated from the other five while they were smoking, or that the other five were not smoking marijuana recently.

These are the roundabout paths of logic detectives need to follow. Individually they are indecisive, but together they lead to a preponderance of evidence.