Decorated pebble in the wall of a boatshed in Evie

I went up to the hide on Burgar Hill. On the loch were teal, mallard, greylags and a single goldeneye. At the end of the loch two ravens were feasting on something. I would bet that it was the remains of a greylag which had succumbed to shotgun wounds- I saw several geese with broken wings from that. Three red grouse flew in and it was nice watching them feeding- usually I just see them disappearing over the hill. Down on the shore I saw fresh otter tracks but no animal. I put up over fifty snipe and almost stood on a jack snipe. Also about: dunlin, turnstone, ringed plover, red-breasted merganser, golden plover etc. I have no idea who put this pebble in the wall.