Mr Cann, 56, was enjoying a week’s holiday alone in Hisarõnü, in southwestern Turkey, in July 2019 when he disappeared.
He was last seen by holidaymakers staying at the Happy Nur Hotel on July 3.
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Concerns grew when he failed to arrive for his flight home.
Later research suggested he had not been to the airport.
Attempts by party officials to contact him by phone proved unsuccessful.
His family was on their way to Turkey within 24 hours of the alarm going off.
Turkish authorities have remained open to the possibility that, for whatever reason, Mr Cann may have simply extended his stay.
The search for him intensified as it became increasingly clear that his disappearance was irrelevant.
Despite numerous calls on his cell phone – until his death – no one answered.
Local search and rescue experts AKUT were called and a detailed search of Mr. Cann’s likely route from Hisarõnü to Ölüdeniz began.
Drones and sniffer dogs were used in case Mr Cann strayed out of the way and struggled in the summer heat. No trace of him has been found.
As the third anniversary of her disappearance approaches, sister Sandra Roberts and her husband, Melvyn, said they were confident everything that could have been covered up was covered up at the time, but the mystery remains.
“Of course, the pandemic has prevented us from visiting Turkey as often as we would have liked since 2019,” Ms Roberts said.
“However, it was reassuring to see a thick file on Dave when we arrived, which showed us that the Turkish authorities were far from inactive.”
“It may be that, certainly more recently, we haven’t had much communication from them, but his case has certainly not been forgotten.”
Melvyn added: “We are aware that speculation about what happened to David is a talking point in local bars, but we see this as wholesome as it keeps his memory alive.”
“We’ve accepted the fact that we may one day have to come to terms with the worst news, but we always talk about him in the present tense and we know that if other people are talking about him too, then he won’t. just be forgotten.
Ms Roberts said questions about Mr Cann’s disappearance were circling and it proved difficult.
He had no history of depression and was an experienced outdoorsman.
Ms Roberts said the trip allowed Mr Cann to relax after leaving his job at the international language school.
She added that he loved his family and that “his life was the best it had ever been” before his disappearance.
“We accept that no one can just disappear; after three years, it seems obvious that something has happened to David.
“The fact that there is no trace of him means we are no closer to closing.
“We came to Turkey both to reconnect with David, to see the things he saw while he was here and in the hope that the anniversary will give new impetus to his research so that we can -to finally find out what happened.”