Usual items are
Alarm - definitely disabled/no power?
Immobiliser (would obv be aftermarket on this if you've got one)
Interior light
Boot Light
Stereo/AMP/ICE not swiching off properly (e.g. someones got continuous live to their amp in error..)

Easiest way is to establish the parasitic drain on the battery and then pull one fuse at a time and see what happens. If you see a sudden drop off on amps being pulled, you know thats the circuit to trace.

Bear in mind there will always likely be some kind of parasitic draw, so don't try and get it to dead zero. But if you are genuinely killing a fully charged 12.6V battery overnight then the cause should be easily found