The brief
The Jetta was losing coolant, the level dropping with a faint sweet smell and a damp patch showing up around the engine. It came up on a service inspection and the owner had it sorted, which is exactly right, coolant that disappears is going somewhere and a slow leak becomes an overheat the day it lets go. When one of these weeps coolant, a plastic connector or flange is a common source. The cooling system on these uses moulded plastic connectors and flanges where the hoses and pipes join the engine, and like all the plastic in there they sit under pressure and heat cycles for years until they get brittle and crack, usually at the neck or around the seal. It weeps a little, the engine heat dries some of it off, and the level keeps creeping down. A cracked connector doesn't reseal, so it needs replacing.
The diagnosis
A pressure test on the cooling system pinpointed it, a plastic coolant connector on the engine was weeping from a hairline crack and bleeding pressure slowly, which is the disappearing coolant. The radiator, the hoses, the water pump and the rest of the system held fine. That's a connector replacement, with a fresh seal, rather than chasing a crack in brittle plastic that's only going to spread.
The work
The cooling system was drained enough to get at it, the old cracked connector removed, and a new genuine VW-spec connector fitted with a fresh seal and the hose clamps renewed. The system was refilled with the correct VW coolant, bled the proper way so no air pockets were left, and pressure tested again to confirm it held with no weep. A road test confirmed the gauge sat steady and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, and the system holding pressure as it should. The Jetta went home with the leak resolved. A cracked plastic connector only splits further, and the failure at the end is a sudden coolant dump and an overheat, so changing the connector and confirming the system kept it to a tidy, planned job.