The brief
The Passat had become a regular for coolant top-ups, the level creeping down between fills with no obvious puddle the owner could point at. Frequent top-ups like that mean the cooling system isn't holding, and on these the suspects are the thermostat, the water pump, a coolant hose, the radiator or the expansion tank. He brought it in for a proper inspection rather than keep topping up. Coolant doesn't go anywhere on its own. When the level keeps dropping, something is weeping it out, often slowly enough that it evaporates before it pools. Run an engine low on coolant for long enough and it overheats, which can cost a head gasket, so a recurring top-up is worth tracking down.
The diagnosis
A pressure test held the system under pressure and showed coolant weeping from the thermostat housing, a hairline crack in the plastic, with the water pump, the hoses, the radiator and the expansion tank all checking out clean. Since the housing and thermostat on this engine come as one piece, the fix is to change the assembly complete with a fresh seal rather than try to patch a cracked plastic part.
The work
Enough coolant was drained to drop the thermostat housing, the old cracked unit removed, and a new genuine VW-spec housing fitted with a fresh seal. The system was refilled with the correct 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 level held and there was nothing weeping.
The outcome
No more top-ups needed, the level holding, the gauge steady, and the leak source confirmed and dealt with. The Passat went home with the cooling system sealed up. A recurring coolant top-up is easy to live with until the day the level gets away from you and the engine overheats, so tracing it down and changing the failed part closed it off for good.