Do you have a water meter? If yes, turn every tap off in the house, see if it's still showing flow. If it is, turn the main stoptap off, see if it's still showing flow.
Is the pressure relief valve continually operating (should be a tundish on the outlet pipe so you can see it).
Is the return from the secondary circulation teed into the cold water supply at/near the tank or does it go into a separate boss.
Do you have a group valve (to equalise hot and cold pressure in outlets where it matters- usually mixer showers), does that group valve incorporate a backcheck valve in the cold supply?
A few pics of the tank & valve gear would be handy, as would a sketch schematic of the dhw pipework.
Is the pressure relief valve continually operating (should be a tundish on the outlet pipe so you can see it).
Is the return from the secondary circulation teed into the cold water supply at/near the tank or does it go into a separate boss.
Do you have a group valve (to equalise hot and cold pressure in outlets where it matters- usually mixer showers), does that group valve incorporate a backcheck valve in the cold supply?
A few pics of the tank & valve gear would be handy, as would a sketch schematic of the dhw pipework.