My view is that on rugged moorland terrain, grouse butts would be a reasonably obvious hazard, and comparable to the hazards presented by natural hollows, peat troughs, boggy areas or even mine workings that can be expected in such an environment. The visitor will usually tend to be the experienced and prepared walker rather than the casual day visitor. I would not necessarily expect to find fencing around remains of old industrial buildings or other built structures on open moorland, although if these are found near car parks or entry points on the edge of access land, or close to well-used popular paths, the situation might well be different. So in most situations, I don’t think there is a need to provide warning signs nor to fence off each butt. The greater risk to ramblers probably arises when the grouse butts are in use for shooting. This link to the Moorland Visitors’ Code may also be useful - http://www.moorlandassociation.org/moorland_visitors_code.asp.