Why would you conclude that?
Well, if they aren't painting or redecorating, they are probably up to no good. This could include drugs, as well as other criminal activities. Or maybe they work the night shift, and sleep during the day.
My first thought is drugs. There is also a very rare disease that makes people supersensative to sunlight. There's a house in my area that has all of the windows blacked out because one of the children who lives there has this disease and exposure to sunlight is extremely painful and could kill the kid.