It’s not a thing in real life either…
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……?
Its hot enough to catch wood on fire… thats why metal ducts have very specific code clearances to combustible’s… yikes… yeah thats the education level on this topic here…? The duct gets a lot hotter than the air coming out of it… try touching an operating one sometime…