People across Northern Ireland as well as those living abroad have been asked to come forward if they have even the smallest piece of information that could help solve this unusual mystery.
In particular, the PSNI are urging anyone who may have had any contact with missing man Peter Bartlett since 2018 to get in touch with them.
Peter, who is from the Belleek area of Co Fermanagh, was reported missing by his family in July 2018 when he was 79 years old.
It is understood at the time he may have been going to France.
PSNI Inspector Brown said: “It is now four years since Peter went missing. This period has been understandably tough for his family and friends.
"We believe that Peter boarded a ferry from Rosslare in the Republic of Ireland to Cherbourg in France on May 22, 2018 and was due to check into a campsite, Ar Kleguer in St Pol de Leon in France, to stay in a campervan he had previously left there.
"However, there were no further sightings of him after this. His family then registered him as a missing person and there have been no sightings of him since his disappearance."
Peter’s family has also released new photographs of him in an attempt to jog the memory of anyone who may have saw or spoke with him since his disappearance in 2018.
He is described as being approximately 5' 8" in height, of stocky build with blue eyes and grey hair.
"We are still following a number of lines of enquiry, including enquiries with authorities in France, in an attempt to locate Mr Bartlett," said Insp Brown.
The police are asking Peter or anyone who knows of his whereabouts to contact them on the non-emergency number 101, quoting reference 948 25/07/18. You can also call Crimestoppers anonymously on 0800 555 111.
"In particular, if anyone has had any contact with Peter on or after May 22, 2018, was on the ferry on May 22, 2018 and remembers him, or have any information as to possible whereabouts for him, please contact police," urged Insp Brown.