Seat Leon Hybrid Fuel Consumption and Hissing Sound

That sounds like a similar issue I had with my Leon. The hissing sound, especially when accelerating, points towards an air leak in the intake system. Have you checked the air intake hose that connects to the turbocharger? A leak there could cause unmetered air to enter the engine, leading to higher fuel consumption and potentially engine misfires.

Did you notice any specific hose being the culprit? I've had a quick look, but nothing obvious jumps out. Is it difficult to spot the leak?

In my case, it was a small split on the underside of the intake hose, near where it connects to the turbocharger. It was hard to spot without taking the hose off. Given your symptoms, it's likely a similar issue. I would recommend taking it to a workshop for proper leak detection and repair. You don't want to risk further damage or potential engine misfire.

Cheers for the help. Took it to a workshop, and as suspected, it was an intake hose leak. Replaced it, and the hissing is gone. Fuel consumption seems normal now, and the check engine light is off. Cost me 100€.