Friends and family carried out searches on Saturday in the Carrigans area after a reported sighting of Martin.
A PSNI Foyle spokesperson said: “We need your help to find Martin Crumlish.
“We are asking all drivers who have a dash cams in their vehicles and were in the area of Foyle Road/Letterkenny Road on Wednesday 8th August between the hours of 11am and 5pm to check their cameras for sighting of Martin.
“Martin is 26 years old and lives in Blighs Gardens, Derry.
“His family have not seen him since last Sunday and we are concerned for his safety.
“Martin is 6ft 1, slim build with brown hair, we do not know what he is wearing.
“If you have any information that will help us find Martin for his family, please ring us on 101 (quote reference number 25 of 10/08/18).”