In the authorized continuation short story The Unravelling, Sergeant Dover is the police sergeant in the village where Miss Marple lives. Dover plays a prominent role in the plot and the descriptions given of him suggests that this village may not be St. Mary Mead but another village to which Miss Marple had moved to.
According to the text, Dover had been the village policeman for many years and knew most of the residents and was fond of them. At one point the text states that he knew Mrs Penny Weaver (now middle aged) since she was a little girl. However the text also states that he did "wonder if things had become rather more difficult when Miss Marple moved into her cottage at the top of the hill." According to the Christie canon, Miss Marple had lived in St. Mary Mead for most of her life. So to have "moved" must mean that she came to this village later, particularly as this story is set around the late 1950s or early 1960s--a wartime infant boy is now 21.
Sergeant Dover intervenes during a seemingly trivial altercation between farm hand farm hand Martin and the haberdasker Mr Weaver. When Martin was found murdered the next day, Dover has the unenviable task of investigating the murder which did not seem to have a sensible motive and which would involve question the popular and well liked Weaver. The crime becomes even more puzzling as the investigation progresses, with unexpected people confessing. Ultimately it is left to Miss Marple to discern what must have happened. Dover has an amiable nature and also some respect for Miss Marple (she sometimes "made him feel as though he was eight years old again, and had been caught stealing sweets). Thus allowed Miss Marple and her friend Susan Goldingay to go with him to the Weavers towards the end for a "denouement".