The brief
Mr Wong's Passat was getting through coolant, needing topping up far too often, so he brought it in to have the coolant pump checked. Coolant that disappears is leaking somewhere, and a leak that's left alone turns into an overheat the day it lets go, which can damage the drive, the timing belt or the cooling system, so it's worth pinning down. The coolant pump circulates coolant around the engine and through the radiator. As it ages the shaft seal and the housing gasket wear, so it weeps coolant, and a leaking or noisy pump tends to make the engine run warm at idle and low speed and cooler again when you accelerate. The right way to check is to look at the radiator hoses first, and if there's no leak there and no pressure holding, that points at the pump. A weeping pump doesn't reseal, and the thermostat lives in the same module, so the two get changed together.
The diagnosis
A pressure test on the cooling system, checking the hoses first, found the coolant pump weeping, which is the disappearing coolant. The radiator, the hoses and the rest of the system held fine, and the thermostat in the same module was due alongside the pump. That's a coolant pump and thermostat module replacement with a coolant flush and bleed, rather than chasing a weep that only gets worse.
The work
The cooling system was drained, the old coolant pump and thermostat module removed and a new genuine VW-spec pump and thermostat module fitted with a fresh seal, the drive that runs it checked and refitted to spec. The system was refilled with the correct VW coolant, bled the proper way so no air pockets were left, and pressure tested. A road test, including time at idle and on the move, confirmed the gauge holding steady, no warm running, and the system holding pressure with no weep.
The outcome
No more coolant loss, the level holding between checks, the gauge steady at idle and on the move, and the system holding pressure. Mr Wong got the Passat back with the leak resolved, under warranty. A weeping coolant pump only gets worse and can take the timing belt or the engine with it if it fails, so replacing the pump and thermostat together kept it to a tidy job.