My battery drains in about 3 weeks.

Hi guys, I have a VW Jetta 2002 1.8T automatic which has 34K miles on it.

I have a voltmeter that I insert in the cigarrete lighter and on a freshly charged new battery with the engine off it reads: about 12.4v.

With the engine idling it reads about 14.3v and if I turn everything on it will read between 12.5 and 13.1v.

But. In about 2 weeks I'll check the reading in the morning before cranking the engine and it will read about 11.9. With engine in idle it reads 14.4v and after I drive maybe 20 miles and turn off the engine it will read about 12.2v. The next day again about 11.8v.

Then the following week especially if I don't drive it for a day or two it will totally drain, not even the door locks will operate.

I don't see any obvious reasons for the battery to drain, e.G, stereo, interior lights. Nothing.

My question is if there is any special procedure or instrument that can be used to try and track this problem?

I would be greatful for any suggestions. Thanks for reading this, Rene Jr. :Idea: