Something could be drawing on it and you might not be able to see it. As Steve said disconnect at night and if battery is good the next day that is a good sign.
The next step would be to disconnect the positive battery cable and place a multimeter between the battery and the positive cable. Then start pulling fuses, when the voltage drops you have found the circuit that is drawing on the battery and can start checking the components in that circuit. When you are going this, if you have an underhood light, take out the bulb, also if you are doing this with your door open.. as you likely would be the inside lights will draw and give you a false reading. Turn them off or go ahead and pull that fuse first and keep it out.