The 38-year-old man from Worthing was arrested on Wednesday a year after Mark Manning's disappearance.
Tip-offs following an anniversary appeal led to a police search at a lake in Worthing, but nothing was found.
The man has been bailed until 25 June while police enquiries continue.
Mr Manning, of Brighton Road, Lancing, was last seen on 19 April 2014.
A friend had dropped him off at Worthing railway station. His family reported him missing three days later.
The 54-year-old was a former military bomb disposal expert who had worked for the Mines Advisory Group, a charity that removes landmines and other dangers.
A murder inquiry began last June, but no-one has reported hearing from or seeing Mr Manning and his body has not been found.
A man arrested on suspicion of murdering a bomb disposal expert who has been missing for more than a year has been released on bail.