The low voltage could be due to the fact the fans are running full blast? They pull a lot of current!
Same thing happened on my MK4 R32. The cause was a duff fan controller. It was bolted to the nearside wing under the battery tray. Do MK5s have fan controller boxes?
If they do, that's where I would start investigating!
The fan controller runs the after run coolant circulation pump as well as the fans (on low speed) when you switch the engine off. When it fails, the most common symptom is what you've got there buddy.