location, location, location

I got this nest box for $1 at a yard sale and gave it to my dad a few years ago - he and my brother put it up and we waited. And waited. 
Today while we were cleaning up leaves I happened to see mr. bluebird going back and forth in that direction and finally realized he was going into a cavity behind the box. 
 I’m wondering if they added a back entrance, or maybe they use the box as a decoy?
Although the bluebirds check out the boxes every year, they have never used them (although we’ve never looked on the other side of the tree).