I just returned to my house (northern New Mexico, ranch-style, built in 1972) after a ten day vacation and found an unpleasant musty smell to greet me. The smell is primarily in the living area and the master bedroom, and I cannot smell it in the attached garage, which is semi-open to the attic. Before I left, I closed up all the windows, so there would be no ventilation other than through cracks in the walls and ceiling.
As soon as I got back, I opened up all the windows and turned on the fans, and while that has helped slightly, the smell is still lingering. The smell is strongest in the area around my bed (foam mattress 5 months old), but I can't see anything particularly wrong there.
Thinking that maybe the water in the traps might have evaporated, I ran all the sinks, but that doesn't seem to have helped. It rained only a very small amount while I was gone (less than 0.25 in). I can't find any plumbing leaks. So I would be surprised if it was some kind of sudden, rapid mold growth. It's pretty bone-dry here right now.
Best Answer
I am guessing you closed all your windows, shut the doors, and made sure the AC was off or set really high.
So then the humidity of the house goes sky high, mold spores multiply, and all of the major smells in your house that were hidden are multiplied. I go on a couple of long vacations a year and my house always smells when I get back if it is warm. Lots of times it leads me to clean something that maybe I didn't notice (steam clean the couch) and sometimes it is me being stupid before I left (leaving dirty clothes out, wet towel on the carpet, left fruit out, half full trash can). The mixture of no air movement, more moisture, and higher temperatures is an incubator for smell. And it takes very little mold growth to get that "old people" smell in your house. Open up all your windows, let the AC chill the place down, and repeat. Takes a couple days to go back.