The brief
The Golf had a coolant pump on its way out, weeping coolant and the level creeping down, caught before it failed properly. He brought it in, which is exactly right, a leaking coolant pump only gets worse, and the failure mode is an overheat that can do real engine damage. The coolant pump circulates coolant through the engine and the radiator so the heat goes somewhere, and on this engine the pump and the thermostat come as one module. As the pump ages the seal weeps coolant and the bearing gets rough, and once it fails properly the engine can't shed heat and overheats. Catching it at the weep stage and changing the module, with the thermostat that goes with it, heads off the overheat. Better a planned job now than a roadside one later.
The diagnosis
A pressure test on the cooling system pinpointed it, the water pump module was weeping from its seal and losing pressure slowly, with the bearing starting to feel rough. The radiator, the hoses, the expansion tank and the rest of the system held fine, this was caught early. That's a pump module replacement, with the integrated thermostat and a fresh seal, before it could fail.
The work
The cooling system was drained, the old water pump and thermostat module removed, and a new genuine VW-spec module fitted with a fresh seal and the drive belt set back up properly. The system was refilled with the correct VW coolant, the air bled out the proper way so no pockets were left, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady through traffic and at speed with no overheating and no noise, and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, the engine warming up on time, and no noise from the bay. The Golf went home with the cooling system circulating properly again, caught before it could overheat. A weeping water pump only fails harder, so changing the module before it let go kept it to a tidy job and saved the engine the heat.