Get yourself a multimeter, set it to amps scale, as high as it will go.
Disconnect the battery earth and connect the multimeter between the earth terminal and the battery earth.
If there is a drain on the battery it will show on the meter in amps, should be no more than 0.15 amps .
If there is a drain disconnect each circuit by removing a fuse or relay to each circuit until you see the drain go on the meter then you know which circuit is causing the problem. If there still is a drain then you should disconnect the alternator and then starter motor as they can cause a drain also.
If the drain is lower than 0.15 amps then it will be a faulty battery.
Hope this helps.