I had a slow fresh water leak in my D 30 Classic which baffled me for ages. That was in the pressurised system in my case, the pump would occasionally cut in.None of the pipewok joints were leaking. It turned out there was a small leak in the body of one of the pipes, a pinprick or a small crack in the middle of a run of pipe in a concealed location, no apparent reason for it, no visible damage to the pipe. I had been assuming, not unreasonably, that the pipes themselves were watertight, but that was not the case in that instance. Replacing that length of pipe cured the problem.So although unlikely, a leak in the body of a pipe can occur.
Mike Simons