@Jem J. A few things:
Check the condensate drain line in "the utility closet with the ac heating unit".
Were renovations done recently (within the last year) to the bath, HVAC, closet, flooring etc.?
Is there an exhaust fan in the shower that runs during showering and for 30 minutes after each shower? Tile and grout are not waterproof --- only water resistant, and that includes water vapor rising from below. Plus, check the draw or power or suction of the fan....single piece of toilet paper held close will should be aggressively drawn to exhaust fan. Placement of fan is important too. I find it easy to imagine vapor directly communicating with the suspect warped flooring above.
'But I've never had this problem before?' Does this new tenant take longer, hotter, soaking showers or baths? Does she dislike the sound of the vent fan running while sipping her Earl Grey in the soaking tub?
Do you know how to check the leak detector on the water meter?
Can you post photos?