I have a 2007 HCH that i just bought, but every now and then while i am driving it, this weired smell, comes out i donno from where, some say maybe from the catalyzer, the smell is like still water that was left for days...
Following on from Mendel's suggestion, you may want to see if there is any moisture in the "air box" where the cabin filter is located. When the air conditioner operates, there is condensation formed from the humidity in the air entering the car. There is a drain which leads to the outside under the car but this can become plugged by dirt, mold, etc. (This is why you will sometimes see a small puddle left under the car when you have been using the air conditioning).
This sounds like it's related to your exhaust system then. The catalytic converter will sometimes produce more odor in the exhaust. The stagnant water smell you describe may also be from condensation in the exhaust which evaporates as the system heats up.
(I notice this when I start my motorcycle after it's been sitting for a while).
I think it would be worth having the entire exhaust system checked for leaks or damage. You should not be able to smell the exhaust when inside the car.