To embarrass myself with a personal example, are you sure nothing is in the bottom of the bag?
I had a sweaty face towel on top of my bag some years ago. Somehow, between the course and home, it fell into the bottom of my bag and I never thought about it. When it got wet in some rain, it stunk like mildew a few days later. Once I realized what had happened, Lysol helped and the towel went into the garbage.
My wife thoroughly enjoyed teasing me, the guy who is so overly anal about taking care of my equipment.
I got lightly rained on the other day and spread out my equipment in the living room to dry everything off. The reason elephants are noted for their long memory is because they learned it from my wife. She just had to remind me about that towel, so I picked up my bag and shook it over her head. I am only still alive because nothing fell out.