HVAC techs will use the anemometer that’s part of their standard kit, they aren’t going to tie a fire hazard that collects dust to a duct….
If you see them left behind, that’s because the company was too cheap to cleanup their garbage, they would ONLY be used while the techs were actively diagnosing a persistent issue, but it should be removed when done.
Why do people perpetuate obvious fake Hollywood stuff? Do you think code will allow you to permanently affix something flammable to a duct that would be heated and can catch it on fire…..?