Lexus GX 2004-09 :: Interior Smells Like Mold When Car Is Running?

This is a 2005 GX470 with 125k miles. We've had this truck for 3+ years and this is the first time we're experiencing this issue.

2 days ago, i noticed that there is a strange moldy smell inside the cabin when i drove the car around town. Not too strong, but definitely noticeable. It was not the usual musty/humid smell that all cars have after you drive them with the AC on for awhile and then you turn it off. This usual smell is common in a lot of cars, i've had it in the Lexus too, and it usually goes away after 10 mins or so.

What i have in our car is a different smell. It smells like there is a piece of old moldy wood or moldy piece of cloth left under the seat for a long time. The smell does not go away. I am 95% sure it comes from the vents on the dashboard.

Today i checked the entire cabin and all floors for leftover food, wet clothes, stains, spills and i found nothing. The car is clean. Carpet appears dry and very clean for a 10yr old car. No strange stains. Nothing inside the car smells like mold, especially when it is not running.

When i start the engine, the smell creeps up. It appears to be coming from the vents. I checked the cabin air filter. It is not brand new, about a year old, but it DOES NOT smell like mold at all. I ordered a new filter but i doubt this will fix the issue. The current filter is not wet or smelly. It's not the source. What else should i check?

There must be an AC condenser drain hole somewhere under the car. Not sure if this is obstructed or not. Where is the AC drain located exactly? Any pointers will be useful.