I had a very similar experience, but NOT at T5. Alarm went off when my son passed thru the sensor, asked my son took off his fitbit, which cost maybe $200. Went thru x-ray, but nothing came out, they stopped the machine and search for good 5 mins, nothing. End up it was, indeed, found on the floor.
I guess "the brush" pushed it down... and I would never think that's their intention to "steal" someone's watch while under CCTV. Especially, in my son's case, it's clearly NOT expensive by any means.

Another time, my wife's cell phone, which I asked the agent to help looking under the machine, again found on the floor.

Since then, I put everything into my carry on bag (if I carried one) before going to x-ray, no more lose item.