If it only happens in summertime, then it's probably humidity in the wall coming in contact with the cold faceplate. It could be poor insulation or air leaks. It could also be a leak in your dryer vent adding humidity behind the walls. Sometimes it's a roof leak that runs all the way down the wall. I've also seen siding nails put a pinhole leak in either a water line or a sewer line.

There are companies out there who can use IR cameras to find evidence of moisture problems in the walls. If this is the only location where you have this problem then it could be a moisture problem behind the walls and the sooner you figure it out the better.

BM me if you have any other questions.