[upthesaukee]

RG, it almost looks like a door into the structure through the yellow forsythias. Could it have been a small observation tower with it's observation deck gone.

the fireplace looks outdoorsy to me, not something that would have been part of a house.

Grab yourself a nice potted plant, pull into their driveway like you know them, go up to the door and knock/ring the bell, and then explain there are over two thousand people dying to know what the heck the stone tower is. Oh, and then tell them "the flowers are my way of saying thanks for the info!"