Scirocco Central

Had my car (2010) for a few months now, today though it did something a bit weird.
Parked it in the garage unlocked, came back to get something out about 10 mins later and I had the drivers door open, suddenly I started to hear a slow repetitive "boing" noise (once every 3 second or so)from what seemed to be the speakers, dash was not lit.
Closing the door semed to stop it and opening it again started it off again.
Looked in the manual and can't see anything obivously related to the door being open, it's not the lights on alarm either as that's a much quicker "ting -ting" sounding alarm that comes from the dash pod - and the lights were set to auto.
I can't get it to do it again either ?
Any ideas

Could be low battery warning my 2010 was very sensitive to it.It has not happened yet on my 2013.

Cheers - yes I think that's it, I haven't been doing many long journeys lately I was expecting it to start complaining soon but it still sounds string on start up.
I have a Ctek battery conditioner I bought for a TT i had a couple years ago, I think I'll plug that on tomorrow.

I immediately thought battery! Take it on a long drive :-)

Have you got DRL's on?