Who touched my sack!!!???

I bring a lunch in to work almost every day. It isn't anything fancy at all. It's a used plastic grocery sack (Paper or Plastic?). In it, I have a microwave lunch, a vanilla yogurt and maybe a Pop Tart. I tie the bag handles together so nothing spills out. So today I head it to retrieve my sack, zap my lunch, etc. I find the contents of the sack, but no sack! Someone stole a plastic grocery sack? I just hope that whoever man-handled my sack, was one that at least washes their hands.