Seat Mii hissing sound on acceleration

Sounds like an air leak. Check the intake hose between the air filter and the turbocharger. I had a similar issue with my Mii. Is the hissing noise constant or only when you accelerate?

Where exactly should I be looking for the leak on the intake hose? Is it something obvious, or do I need to do some kind of leak detection?

It was pretty obvious once I looked closely. A split in the rubber. You could try spraying some soapy water on the hose while the engine is running and look for bubbles, but honestly, it's probably easier just to take it to a workshop. They can check the whole air intake manifold system properly.

Turns out it was the intake hose like you said. Replaced it, cost me about 100€. Engine light is off and the hissing is gone. Fuel consumption seems normal again.