It could be a odor from the AC or vent outlets. This can be caused due to letting a car sit out in a rainy area over time. I understand a mold can develope in the bottom of the fresh air inlet. The dealer should have a chemical spray they use to remove it. Then run the vents (open) on the highest fan speed to clear out any left over chemical odor.

Looks like if they were a way to spray Purex down into the fresh air intake, that should do the same thing. The fresh air intake is located under the windshield wiper area. Their is a water drain under the car for the fresh air intake. Some have a rubber flap to close over the opening when their is no water present. That outlet can stop up where any water can't escape and then causing a odor.

 

You can buy two types of cabin air filters. One has activated carbon in it, and should look darker grey or black. Activated carbon physically traps very small particles that are in the air which may be causing an odour. It is used in water to remove taste (which in many cases is really a smell).

However, this is really a coverup approach. If you left an egg salad sandwich under the seat, an activated carbon filter is not going to do much in the long run.

And of course do not run your heating and air conditioning system on recirculation. Fresh air from outside is brought in when you are not in recirculation mode.