The prime minister's chief advisor travelled to County Durham in March to self-isolate with his family while official guidelines warned against long-distance journeys, apparently because he feared that he and his wife would be left unable to care for their son.
Barnard Castle is around 30 miles from where Mr Cummings was staying, meaning if he had travelled to the town on April 12, it would have been against the lockdown restrictions.
The sign on the wall in the town reads: "DOMINIC CUMMINGS WAS LURKING IN THESE PARTS, 12.04.2020."
Blue plaques are installed in public places to commemorate a link between that location and a famous person or event.
Following the allegations made against Mr Cummings, Durham Police has been asked to "establish the facts concerning any potential breach of the law" following a complaint from Robin Lees who told the Guardian he saw the aide in Barnard Castle.
Mr Johnson fronted Downing Street’s coronavirus briefing to back Mr Cummings on Sunday, saying he had “acted responsibly, legally and with integrity” and that “any parent would frankly understand what he did”.