When I first bought our Discovery coach with the same engine, the previous owner had the coolant flush performed just before the sale. I also experienced a low coolant warning light and put in exactly one gallon of antifreeze/water mix. But unlike yours I never saw anything leaking, and after putting in the gallon I never had to add any more.
During the flush the mechanic is supposed to turn off the heater valves on the engine and turn them on in a certain sequence so all water in the heater circuit is replaced and no air is trapped. It is possible the valves leaked a little when they were turned off and on since they aren't used very often.