That could be the problem. Some stereos are wired up so that they constantly have a power supply, but don't turn on until the ignition is turned. If it is wired into the constant power supply wires then the boost gauge could be be on permanently. Even if it doesn't suck much juice, over time will drain the battery. Take a multimeter to the cables in the back of the boost gauge and see if any current is flowing through it, no matter how small, when the car isn't on. If there is a reading then this could be the problem.