Sometime over the Christmas holidays, when the cookie jar was emptied and cleaned, it got closed, sealed, and put away when it wasn't completely dry inside. I didn't notice until I was ready to fill the jar again a month or more later.
The mildewy smell was evident as soon as I opened the jar. So, I washed and dried it and let it air out for a day or so. Then, I put what was remaining of the cookies in the jar. The next day, no one except Doodle could stomach them. The cookies had taken on the mildewy odor.
So I scrubbed the container again. Still stinky. I tried bleach. No help there. I thought the rubber gasket might be what was harboring the smell, so I soaked it in bleach. That still didn't help.
Next I tried soaking the gasket in a vinegar solution. That did the trick for the gasket! So I then filled the cookie jar with a vinegar solution, clamped on the lid and let it sit for a day. The jar now not only smells fresh, it look sparkling clean as well.