A slow, mysterious coolant loss with no obvious puddle — and a sweet smell after a drive — on many VWs is a cracked plastic coolant flange / connector (the plastic elbows and temperature-sensor housings that VW uses around the head and block). They go brittle with heat and age and split at a seam or an O-ring, and because they hide low and behind things, the leak is easy to miss until the level drops
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