Musty smell under bathroom sink

[Alexandra D Pufan]

I've put a fan under there for a day so nothing seems wet (the board & wood are dry to the touch). Does anyone know what could be causing this smell?

 

[Alexandra D Pufan]

Added two more pictures.

 

[Sponsor]

 

[Reach4]

Try separating your source candidates with plastic sheet and tape. The next day, sniff which side of the plastic smells bad.