There is a procedure to check battery parasitic drain. It has to be done properly to be meaningfull. It involves using a low-reading ammeter inserted between the battery negative post and the removed cable. All doors must be closed, ignition key removed, and a period of time must elapse (ie 20 minutes) to allow modules (Ie transmission, body,etc) to power-down (draw minimal power). The current draw should be ABOUT 0.03 amps. If higher, fuses can then be removed and the effect on the guage noted. A drop in amperage indicates that something in that fused circuit is drawing power when it shouldn't. Example: trunk light, glovebox light, alternator diode, etc. But there is procedure - you can't just remove the battery cable and insert an ammeter. There are detailed procedures with illustrations on the internet. A garage should have experience in doing this.