OK, scratch that.

The problem was gone, I drove around for 20/30 minutes and went for some gas, came home - no problem at all.

Then I washed the car and put it inside - waited 1 hour at home then took the Mrs shopping, and now the problem is back. 12v at the dash meter and a very dimly lit-up battery light.

The problem started just after a downpour that left the streets flooded to knee depth at max. The car was parked outdoors throughout the storm, and I drove a bunch of kids home after their very wet football match through the floods.

When I washed the car, I didn't use a jet-washer or go under the arches etc - just a small hose from a domestic tap... washed the car from the top down.

Could it be water ingress causing a problem somewhere?