I can find no visible signs of wetness in the floorboards, but there is a definite smell of mildew lingering a couple days after the car is driven in rain.

Please advise on probable cause and remediation. Thanks!

Sunroof gutters being clogged causes this, doesn't it?

I'd make sure that water is draining properly in that system.

doodlebro wrote

Sunroof gutters being clogged causes this, doesn't it?

I'd make sure that water is draining properly in that system.

What he said, sunroofs are just a liability

I am guessing you drive the car with the de-fogger running when it rains. If so, you might just be experiencing the "aftermath" of putting the car away while the A/C was running. That alone will create a musty smell next time you drive the car without the A/C running.

Try changing the cabin filters located in glove box