Mark Manning, 54, of Brighton Road, Lancing, was last seen on 19 April 2014 before his family reported him missing on 22 April.
His disappearance has been described as completely out-of-character.
Detectives launched a murder investigation last June. His body has not been found.
The former military bomb disposal expert worked for the Mines Advisory Group (MAG), a charity that removes landmines and other dangers in some of the world's poorest countries.
Police said extensive proof-of-life inquiries showed that the father-of-one had not withdrawn any money or used his mobile phone since he went missing.
No sightings
Mr Manning lived alone in Lancing but was very close to his family, particularly his son.
Police said he had not turned up for pre-arranged appointments and meetings, and there had been no reported sightings of him.
Det Ch Insp Mike Ashcroft of the Surrey and Sussex Major Crime Team said: "We want to hear from anyone who may have seen Mark or has any information about what has happened to him.
"We are determined to establish what has happened and to provide answers for his family, who are desperate for any news about him."
Mr Manning is described as white, 5ft 9in (1.75m) tall, clean-shaven, with a tanned complexion and of a stocky build. He has blue eyes and is bald.
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