Out of Nowhere

I have a problem, and I need your help.

Shortly after returning from my China trip, I began to notice something. Everytime I stepped into my apartment, I smelled this smell in my place, basically when I left some clothes undried after washing. And then it basically starts to grow stuff. I liken it to a wet-grass smell, and it's gross.

That's the problem, and my bigger problem is that I can't find the root of that problem: I have no idea where the smell is coming from! For the past few days, I've been sniffing around like a bloodhound in every nook and cranny of my apartment, and I can't find the source.

I've checked all my laundry -- dried and smelling nothing like wet foilage. I've taken my couch apart, and vaccuumed all through and under it. I've sniffed through the bathroom, the kitchen, even upstairs in the closets (in case there was a leak and somehow my clothes got wet). Nothing. I checked under the kitchen sink, around the bedroom, under tables and behind the TV. I opened every umbrella and took a good whiff. I stuck my nose in every shoe I own. Nope.

Where the hell is that smell coming from??

I can't figure it out, and it's driving me nuts. I even went to Carrefour last night to buy cleaning agents for the air conditioning unit, in case it's coming from there, but I doubt it, since I believe the smell to last even after running the AC for some time.

So far, I think I might have localized it to around the couch or fridge (just beside it) or the aircon (directly above the sofa). I just can't seem to get any closer than that, though. Ugh.

Worse yet, each time I come home, I only have a few minutes to do my nasal research before my sense of smell acclimatizes to the scent and filters it out in my head.