I haven't set it off at Denver, but just set it off at SFO. (Precheck/Clear queue.)
I always divest my wallet and wristwatch. The belt was the culprit at SFO, but I was wearing the same one at Denver, Phoenix, and Los Angeles.
It was a "long beep" at Cleveland, so that was a random.

Although one could argue that this is "inconsistently inconsistent," I would argue that archways are imperfect.

I am convinced that the metal detectors at YYZ will respond to iron inside red blood cells. Pro tip: donate blood weekly, and do not consume any meat, beans, tofu, etc. at least six months prior to travel.