The brief
The Passat had been creeping toward the hot zone in slow traffic, and the owner had been topping up coolant every couple of weeks. He brought it in before it overheated outright.
A gauge that climbs in traffic, when there's no airflow through the front, plus coolant that keeps going down, points at two things at once: the engine isn't holding its coolant, and it isn't regulating its temperature properly. On this engine the water pump and the thermostat are part of the same module up front, and both can age out together, the pump weeping at its shaft seal and the thermostat going slow to open. Either alone is a problem; both together is an engine on its way to an overheat.
The diagnosis
A pressure test located the leak weeping at the water pump shaft seal, and live data showed the thermostat slow to open fully. Two failures stacked together: a leak that was draining the system, and a thermostat that wasn't regulating cleanly.
Replacing one without the other would just delay the next visit, and since the pump and thermostat housing on this engine are one assembly, the sensible fix was to change the whole module with fresh seals.
The work
The cooling system was drained, the old module removed, and a new VAG-spec assembly fitted, water pump and thermostat housing together, with fresh seals. Then the system was refilled with the correct coolant, the air bled out the proper way, and held under pressure to confirm it now held.
A road test, including a spell in traffic, confirmed the gauge sat steady and there was nothing leaking.
The outcome
Temperature gauge steady through traffic and at speed, no level drop, and no warning lights.
The Passat went home cool and dry. A weeping pump and a tired thermostat only get worse, and the failure at the end is an overheat that can cost a head gasket, so changing the module before it let go kept it to a planned job.