that's normal, it's mostly water coming back through the turbo water return and as the turbo and manifold were so hot and are bloody great lumps of steel, they hold enogh heat to boil the water for a good while.

presumably the last 10 minutes before switching off you drive the car without boosting, or let it idle a while to cool the turbo a bit before switching off?

it's important to cool the turbo by water, otherwise the oil that's in the plain bearing on the turbo shaft can become burnt on and redce the life.