"roughed out in drystone often, to test alignments and appearance. Sighting posts may have been made by this method and would leave virtually no archaeological 'signature'"

You could also speculate whether some stone monuments were long preceded by wooden ones. Replacement in stone would obscure the signature - except, perhaps, in the case of your now-removed sighting stones. In those cases there is now no pointer as to where the original postholes should be sought.
OK, it's a speculation on a speculation but there are so many postholes being discovered "by chance" that there might be some really significantly aligned ones still to be found.