I think the 3rd level assessment of Team Desir is a guess based on either the approximate strength of the defeated chimeras or from a report written about the two real inspectors that D&R are impersonating. If there had been a level scanner in the world, it wouldn't have been a good decision to introduce it in a past time line and not still have the tool in the present.
Well, that's why re-reading is good. Turns out a few chapters ago the story introduced mana detectors from 120 years in the past that can transmit the locations of mages and their approximate mana capacity onto a screen. And during the Team Dungeon arc, individuals are shown to be able to use a spell for similar mana tracking. I suppose Desir's equipment has a large enough mana reserve to fool the detector into thinking he's a Circle 3.