Raypak Heater Leaks with "No Demand"

[SI67]

Our pool has a gas-fired Raypak P-R460A-EN-C heater. When the pump is started, the heater is fine while it is "Heating." Once the set water temperature has been reached and "No Demand" displays on the info panel, a significant steady drip of water starts, It comes out of the heater at the lower right edge. I can't tell where the leak originates.

So what happens when the heater is in the "No Demand" condition. Of course the gas burner stops, but does the water flow path change from what it is was when "Heating" was in progress? Does the water go through a bypass when we're no longer heating? That's the only way I can think of to explain why the leak occurs only in the "No Demand" condition.