I fitted a new battery to the Passat. It has sat unused for about two weeks. Went to start it just now and its pancake flat. No instuments on the dash are even lighting up. I'll have another look tomorrow to find out whats up, but am unsure where to start. Any suggestions?

My first bet would just be that the battery although new was not fully charged when you bought it and the past two weeks cold temps and the cars security system has flattened it out.
Fialing that check out such things as boot lights remaining on or more likely the starter motor switch rmaining in the open position leaking to earth.

Try the boot light first. Have you got an amp meter? If so connect it to the battery and check the readings and remove each fuse one at a time til you find a suspect

What taff said.

Underlying fault that cause the original battery to keep going flat?

I'd get it jump started, (bumping probably won't work if its that flat) and then see how it fares, as above it might just need a good charge.

Don't leave it flat, take it off and get some charge into it. If it's left flat it can cause damage to the cells that is irreversible. I'd take it back and say its knackered and start again with a new one. It may not reach full charge capacity now its been left flat for so long. It could of been duff to begin with.

Car battery draining. Any tips to find out where?

Small hole in the casing.

Certainly, I didnt run it after it was fitted. So it may just need charging. I'll recheck all the interior lights. I wasnt aware the security system could drain it so much. I think the old battery was just toast. I could run it for a while, thinking it was holding a charge, but it wasnt. I checked the alternator output, it was fine. So I then changed the battery. Its been uber cold here so it would have had an impact on it.

two weeks on a near dischrged battery with the factory security will kill the battery off-had it happen on my passat and works picasso

It should have had a charge in it when you bought it. Car batteries don't like being flat – it buckles the plates. Anyway, try this: find where the earth strap connects to the body (probably with a screw) – undo it and make sure there's a good connection. Often there is rust in the way which prevents this. Generally make sure all connections are good and then cover with vaseline/squirt with WD40.