Let it go....

next time you ask him to do work for you.... give instructions that you will pick it up when it is done. or if you agree for him to drop it off, then let instructions ahead of where to put it.  

Look at this as an opportunity of what people will see if/when they drop by your place. If you don't want people to see your place, look at this as an opportunity to do something about it.

edit:  after reading responses from others.....  I don't think this guy was up to No Good.  Some people are just Nosey. They like to snoop around in other's persons places, and things.  I think this is what he was doing.  If you don't want people snooping around, you need to keep things closed up.  It's on you to take action.