Slight nitpick, but if someone got tired walking over every time the bell rang, then surely the bell must have rung several times, in which case the scales must

Slight nitpick, but if someone got tired walking over every time the bell rang, then surely the bell must have rung several times, in which case the scales must have picked up several defects, not zero. You do also need to factor in the electricity costs for the desk fan, although it probably still is a lot more economical than the $8 million solution.