My "normal" Golf made a similar sound when I unlocked and opened the door. Maybe this happens on more than hybrids.

rcomeau

 

FWIW the power steering system is an electric servomotor, it does not use hydraulics at all. The brake system uses an electrically powered "accumulator" to build up a reserve of pressure. And yes, even if the accumulator fails, you will still have manual hydraulic brakes, but a LOT of pedal effort will be needed to stop the vehicle.