Hmm. You might be right. I do recall reading about air sampling tech used to monitor for bio-terrorism attacks in large cities.
https://fas.org/sgp/crs/terror/RL32152.html#_1_3
It would not be something you could construct, but you COULD find one in a lab, hospital, or maybe a military base.
Could be used to see if someone is emitting fungal spores or other bio-indicators of a fungal infestation. But it wouldn’t be something you would find and use initially, because I doubt a piece of pre-cataclysm biohazard detection gear would work on the mycus by design. Maybe have it be an item you go find for him so he can make adjustments to turn it into a mycus-detector.