Bicycle detail

On Sat. afternoon at about 2pm, I locked my bike to a signpost outside the Perkin's restaurant within view of the main entrance, I believe by attaching it as I have done previously to the centre frame of the bike and then to the pole. The pole was attached to a concrete block, so I thought it seemed pretty sturdy. When we returned to the spot around 3:10pm, just the lock was sitting there at the bottom of the pole, but it was fastened! From inspecting the scene, the concrete block appeared that it might have been rotated a couple inches recently, and there were a couple small pieces of black plastic nearby, that might have been from the fender.