Thanks to a posting by @AnnB I have found an entry for my dad's cousin who died in WW1. Walter Frederick Carrett[e] is listed CWGC as KIA 23/3/1918 France and Flanders and he is remembered on the Pozierers Memorial (so no body recovered?) This is the Red Cross entry for him. Do you think the dates are when they searched the pow camps for him? I cannot think of another explanation.