If you sweep with disinfect tool often the tool will set a disinfection as required when there are zero germs, see screenshot. The sweep and screenshot were all done while the game was paused in the same instant.
This appears to follow dupes ladder pathways so it would seem to be a result of a state triggered by their passing, possibly germs in small numbers, which have since vanished.
Once germs have vanished from a square I would expect disinfect not to be triggered. It looks like the squares are not resetting their infected state flag to "not infected" when the germs have died off, or something like that.
I believe this happens subsequent to a minor contamination when a small number of germs have been present and rapidly died off.