[chittychittybangbang]

Check for coolant migration first. If it's dry then poke a hole in the plastic to let any pressure leak out there as shown in the FAQ section. I don't know why but there was a mix of parts so it can affect up to 06. Even though the Audi/VW Passat coolant tank is a completely different design, it also can get coolant migration. It's partly VW's fault and partly the parts supplier fault.

 

[chittychittybangbang]

Unfortunately I don't think it was fixed after 2002 - either that or it's more rare. Some mk5 had the old style tanks but I'm not sure if they really did fix it or if they were just using up old tanks they had lying around. Some Audi/VW coolant tanks got it too and that's with a completely different tank design. In any case it's a pretty rare problem but if it does happen it could total the car.