"I'll repeat that. They didn't detect at the (non-standing) stone."

Had it been scheduled they were certainly likely to have been detecting within the boundary of the designated scheduled area.

"That the stone was shown in a flashback having once been erect was unknown to any character in the series."

Had it been a real happening that wouldn't be accepted as an excuse. Detectorists are supposed to check there are no scheduled monuments on the fields they are detecting. In addition, "permission" can't be given by a landowner to detect there. And in addition again, if he did then he'd lose any payments from Natural England, a condition of which is not to allow detecting on Scheduled Monuments, Sites of Special Scientific Interest and known archaeological sites on Agreement Land. NB, "known archaeological sites" not just scheduled ones.

I can see I'm being thought of as a spoilsport over a gentle, lyrical programme and defences or excuses are being offered on behalf of the (fictional) characters. So be it. Someone has to point out what's right - that implying there may be treasure under standing stones to an audience of perhaps thousands of people who are thinking of going out tomorrow to buy £25 Tesco's detectors is a damn bad idea.