My first thought was termites as well.  Termites don't like light so if they were able to get into that box from behind you'd never see it. 

I learned a lot about termites one year when I was home from college.  My parents home had been infested with them and treated by Orkin, along with a "guarantee". A few years later I was looking in a store room and found evidence of termite activity.  Dad was busy and told me to make the arrangement to have Orkin back out.  After the "local" rep came out they tried to saw this was a new colony and they would have to retreat again but not stand behind any new damages.  I was 20 at the time and while I was inexperienced in those types of negotiations I knew something didn't add up.  I got a little more aggressive and told the guy if he wasn't going to take ownership of the issue I wanted to speak to his superior.  A week later I had the state supervisor at the house.  He tried to give me the same story.  I'd done some research in the mean time and simply asked how long their product stayed active in the soil and recall using the term "half life".  That got his attention real quick.  He told me to get a quote to repair the damage and they would retreat the next week.  I believe they wrote my parents a check for all the repairs which was well over $20,000.