Battery won't hold charge

[mgh]

I have a 16 plate A4 TFSI 1.4 petrol engine with a flat battery.

One day into a 3 week holiday we get a text from a neighbour to say that we had left a light on...

It was the parking light (e.g. indicator was on when the car was last stopped).

No problem I thought - will just jump start the car when I get home and all will be fine - ha!

When I attached the cables the car started just fine - took it for a 30 mins drive, but as soon as I hit the stop button all power was lost. Couldn't even put on the hazard lights.

Jumped again and it started and ran just fine, but same problem  - as soon as stop button pressed all power is lost - can't even put on a light, parking brake, open the boot - anything.

Called green-flag and the guy measured 15 odd volts across the battery when engine running - so alternator is charging the battery just fine.

Surley leaving a light on wouldn't destroy an audi battery that is just under 3 years old?