The Ouest-France newspaper reported that witnesses had spotted a man resembling Xavier Dupont de Ligonnes, leading police to raid the religious site in Roquebrune-sur-Argens in the Var region.
Dupont de Ligonnes was the subject of an international arrest warrant in a case that captivated France.
The businessman from an aristocratic family is suspected of murdering his wife and their children in the western city of Nantes before burying them under the terrace of their elegant townhouse.
Each had been victim of what French prosecutors described as a "methodical execution", with two bullets fired from a silenced weapon at close range into their heads, before they were rolled in lime and buried under cement.
Dupont de Ligonnes told his teenage children's private Catholic high school that he had been transferred to a job in Australia.
He allegedly told friends he was a US secret agent and was being taken into a witness protection programme.