Possibility (a) from Golf with Rain Problem:
► Moisture completes a circuit somewhere, causing the battery to drain.
If this is the case:
1. It should be possible to jump start the car and/or recharge the battery, and then get it going again once it's dry. And indeed, it is possible to jump start the car.
2. Narrowing down where the circuit gets completed would be the project. How can this be determined?
► The battery could be damaged in some way that it doesn't hold a charge long enough to start the card.
if this is the case:
Battery tester. 10-12 dollars. napa 
Inspect the fluid levels in the battery. The plates should be visible in every cell and fully covered by fluid. Use a flashlight. Add distilled water if required.
Test the specific gravity of the fluid. This will establish the degree of charge. Driving for 30 minutes should show as substantial charge. Drive further. Test again. Test to see if and how fast the battery discharges when parked.
There are many types of integrated battery testers, each one corresponding to a specific condition testing procedure, according to the type of battery being tested. Their common principle is based on the empirical fact that after having applied a given current for a given number of seconds to the battery, the resulting voltage output is related to the battery's overall condition, when compared to a healthy battery's output. wikipedia google