They didn't detect at the (non-standing) stone.
I'll repeat that. They didn't detect at the (non-standing) stone.
That the stone was shown in a flashback having once been erect was unknown to any character in the series. There was nothing to say or even infer that it was a "standing stone" in the series. It could have been a depiction of an old cattle rubbing post, a waymarker or a stump of a fieldgate.
That was what was so funny. Like the climax of series one where the shot tracked to the soil a little deeper below where they were detecting to reveal all the unfound Saxon Trinkets. It is a lovely gentle, harmless touch. Great intentions, commitment, self-taught historical knowledge and informed conjecture from two beautifully drawn, flawed characters - but they pack up for lunch a few yards from a pile of treasure.
Then don't go back. Brilliant.