So, to be clear right from the start, there will be no answer to this one, ever. (If it is a weird sex thing, there is no way my parents-in law would tell us - it would be too awkward!) This is purely for speculation, but what are they up to?
Every Sunday evening my parents-in-law's neighbour creeps out of his bungalow and down the path beside the fence to hang a home made wooden shutter over the frosted glass window of their bathroom then heads back inside. This has happened every Sunday evening over around the last six months. If my parents-in-law are in the garden, he waits until they have gone inside but when they look out again later, the shutter is in place. (If they are still there at the critical time he actually ducks down below the level of the fence so they can't see)
What can they be doing that needs a wooden shutter on the window? The most innocent explanation that we can think of is creating a dark room but in that case, why try to do it so secretly and with such a regular schedule?
Please share your ideas because we are stumped.