probably coming through an exterior wall

I'll bet your gutters are full of wet, hot leaves or spring tree litter. Sometimes I f you have an attic fan the vacuum it creates pulls air in over the eaves and the smell with it.

Good luck

Check the floors and the AC vent.
Is the bedroom carpeted? Is there any dampness?
Have you checked the AC vents? When was the last time you replaced the HVAC filter?

Smell your windows. With all the rain we've had, some windows are hurting.

For the past week, I have noticed a mildew smell in the doorway to the master bedroom, which is on the upstairs level of our split foyer. Can you help me identify where it might be coming from? Here's what we've checked already:
-Looked in attic, no leaks from the roof (even went up during a torrential rainstorm, all is dry!)
-No signs of dampness in ceiling or walls adjacent to doorway
-We unscrewed all electric plate covers on those walls and sniffed behind the walls, no smell or signs of moisture in walls
-No signs of moisture in rooms below the master bedroom doorway, which I believe would rule out any pipe leaks
-We run a dehumidifier in the guest room, which is in the basement and directly below the offending doorway
-It's definitely not an errant piece of forgotten stinky laundry or the hamper

It's so weird! Any ideas what might be going on here?