Useful links
This seems to validate that moisture is shorting a component. But it offers no detail. Yahoo Answers 
Here is an overview of how to find a "parasitic draw" (note the term, useful for searching?) 2carpros.com
General overview at itstillruns.com
A discussion in which the owner of a 2002 Passat (similar car) is seeking a parasitic draw; experts seem unanimous that he should test the battery (which aligns with Ward's suggestion). passatworld.com
Less useful links
I *think* we can rule out the failing coils, distributor cap, etc. issues that are discussed on some sites, because those seem to involve arcing electricity when the ignition is cranked. That's not the case here.
A forum post about a Chevy (focuses on distributor cap and ignition system)
Probably useless chat from cartalk.com
Very useful video for probably a different problem (starter cranks but doesn't turn over): Distributor Cap video