2002 Volkswagen Golf Just Bought Car Battery Keeps Getting Drained Overnight

You need to do a currant draw test, disconnect the battery pos(+) cable and fit an Amp meter between cable and battery, with key out and doors closed read the current draw, it should be around .1 .15 Amps max there abouts, if up around 2 or 3 start pulling fuses one at a time to see which circuit drops the current draw the most, and also unbolt the main alternator cabal as if you have a blown diode in the rectifier this will cause a draw down as well but still give you a charge rate all be it down in capacity.