What is stale moldy smell coming from inside air vents also no water drainage un on 2010 Chevrolet Malibu

No ac water dtains out under car after ac use.

I had a bad smell in my Malibu the first year when I had the air vents open, no air condition or heat was on. I contacted the Dealership and they said to spray a can of Lysol into the air vents and then turn the car on and turn the air vents on high and let them run for 20 to 30 minutes. I did this a couple times and the smell finally left and I haven't had a problem since. They said something in there is plastic and it will have a smell. Unreal!