Any chemists/biologists to explain to me this weird thing that happened?

[Twimm]

So, awhile back I found some old papers that had gotten wet and then suffered mildew damage. Since they were sentimental, I packed them in a plastic bin (the lid was on but it was not airtight) and put a liberal amount of baking soda in with them. Then the bin sat and I forgot about it for gosh, nearly a year!

 

Fast forward to today. The bin was not opened till now, and has been stored at room temperature, not exposed to water (although I live in a pretty humid area and its been a muggy summer). When opening the bin, a lot of mildew smell is gone, the papers are bone dry, and the baking soda of course has gotten everywhere. This is the weird part, though - on the outside and top of the bin, not the inside, are droplets that appear like beaded condensation, of a clear liquid that feels mildly greasy. It has dripped onto some of my other things, though it doesn't appear to have caused any damage beyond spreading a faint odor. Weirdly, it has a mild vinegar-y smell, sort of tangy but not sharp - it doesn't smell like the mildew at all.

 

What odd chemical reaction is causing this? I have a zillion of these bins, and this is the only one doing this, so I assume its some sort of reaction caused by the baking soda. Is this water, or chemicals from the plastic, or something else entirely? Should I be worried about its toxicity? From what little I remember, baking soda is a base, so is this acid from somewhere fleeing the baking soda on the inside of the bin? I don't have enough knowledge, but I'm fascinated.