Many people and racers don't realized that the current out of all batteries flows form the negative post to the positive post, the negative post is the hot one hence the need for very good "grounds"
You can test your system for current drain by disconnecting the negative cable and use your multimeter as a jumper wire from the battery post to the cable in the Volt reading position Some techs use their eyes to see if the battery will make a small or large spark when reconnection either cable back onto the battery also