I had a hippo in a drawer that had the most mold in it

I don't know about the air-con's age, I'll have to ask the landlord about that. It looks newer to me.
As for the apartment, it's big and bright and we do not use much air-con, and even when we do it's not much lower than the outside temperature, just enough to get rid of the moisture. Also we don't have much stuff, no rugs and moisture trapping things, chest of drawers in each room, and open clothes racks.
Last few days was so humid, aircon couldn't get it down. My guess is that mold was already there when we moved in, dormant, waiting for a perfect alignment of the stars.