Mold; if I can smell it, why can't I see evidence?

500 sq. ft. basement finished out about a year and a half ago. Reverse grade under the deck where the dog dug out a cool spot and sprinklers near another ext. wall. No sign of moisture intrusion at ceiling, wall (poured concrete) or floor (slab). A strong musty smell like long-term moisture damage w/mold present.]
On this level, a bathroom with a shower has no ventilation. About three minutes of the shower running caused the humidity to rise from 24% to 31%. Long showers a possible source?
With a smell this strong I would expect to find visible moisture damage in materials that have been in place for a year and a half if the source were groundwater/ runoff.

I thought that the smell mold gave off was a gas excreted as mold fungus consume cellulose or lignen or whatever they’re eating. If they’re active and eating, where’s the evidence of moisture? If there is none and they’re inactive, where’s the smell coming from?

Recommending invasive investigation by mold mitigation co.

Mold is often found behind bathroom walls. You described severe humidity conditions in the bathroom. I don’t test for mold, but I would have used my moisture meter to try and locate a possible moisture saturated area. I would still call it out for mold testing.

Erol Kartal

If there is no visible mold growth, then the best starting point is air samples. The outside air should be compaired to the indoor air to determin if there is an elevated mold condition indoors. If there is, then the investigation can get hairy. Is there central humidification? Forced air furnace? Ample fresh air being introduced?

I wouldn’t really consider either 24% or 31% severe humidity, Erol. was just wondering if the shower could have contributed to mold growth.
I tested the entire bathroom floor and found only very slightly elevated moisture levels.

If the moisture source was related to the bathroom, I’d have expected to find elevated moisture levels in wall, floor and ceiling materials near the bathroom. Nope.

If the source of moisture were groundwater/runoff, I’d have expected to find elevated moisture levels in wal, floor or ceiling material near exterior walls. Nope.