Lunch bags

I opened up my lunch bag to retrieve my yogurt this morning, and caught a whiff of a smell that seemed somehow familiar. I'm assuming I know it from some other part of my life when I routinely carried a lunch with me. . . I don't spill stuff in the bag. It's totally clean. & anything I put in it is generally already packaged or in a ziploc.

Why do insulated lunch bags smell like that?