To trace a current drain, you say a light is not useful but a voltmeter is. I would tell you the opposite.

An ammeter is what is needed, not a voltmeter. When someone uses a voltmeter and quotes things like "I have a six volt draw", he is sending a message electricity is mysterious to him and he knows little about it. The meter you have is next to worthless if it is used in that fashion, for sure.

If you know your test lamp, it is far more useful. You must know how much current it uses to light it at battery voltage. A good test lamp might be a dome light, 12 watts, or a taillight, 5 watts. It would light brightly with a drain that would kill a battery overnight, where your "voltmeter" method would indicate 12 volts "draw" if even the glove box was left open, a 0.1A load.

A smaller test lamp, like those sold for that purpose, may only be a 1 watt bulb, but you will be able to tell the difference between a serious drain and the trivial keep-alives for the radio and clock. The key to each method is learning the use of it before being called upon to need it.