We Know Your Secret - Bishop and Kelly, #2

A couple out for a walk on cold January morning discover the body of a man who appears to have fallen into a fire next to his tent on the beach. The fire brigade are called and the police notified. Within an  hour a murder investigation by the Dorset Police Major Crime Investigation team led by Detective Chief Inspector Mike Bishop has begun. The body is that of James Garland, a local businessman and keen night fisher who has been the victim of an apparently motiveless crime.
There is little in the way of forensic clues, other than that he has been shot by a high powered rifle and a message painted in green on the interior wall of one of the shops he owns 'We Know Your Secret.'  In March the body of Andrew Dunlop, a schoolteacher, is discovered in a Dorset field by a schoolboy who is flying a drone, Dunlop has been killed by a high powered rifle, forensic tests prove that the weapon was the same one used to kill James Garland, there is a message daubed in green paint on his cottage wall 'We Know Your Secret.' In June, duing the Covid-19 lockdown Millie Hepburn, who had witnessed a murder the previous summer disappears and becomes the subject of a murder investigation.
In September Craig Hughes, a former Army veteran, now suffering PTSD and living in a former RAF Radar station is found dead, he has been killed by the same method and weapon as Garland and Dunlop, a 'We Know Your Secret, ' message is found. What is the link between the murders and what is the secret they have been keeping?  Why did Mille Hepburn disappear for a week only to be discovered in a disused former lifeboat station?DCI Bishop, DS Kelly and the team are back.