What could cause the battery to be drained when the car is left sitting?

with the car off and the battery fully charged use a voltmeter on the battery should read just over 12 volts,if it reads less than 12 volts have someone pull the fuses one at a time,and observe the voltmeter,when the fuse is pulled from the faulty circuit the draw is eliminated and voltmeter will 12+ volts,this will identify the circuit that is drawing on the battery,further diagnostics will help find and repair the problem.its important that the battery is fully charged to begin with.

There is a drain somewhere. Maybe a dome light or map light?