You may have a drain that is so low it won't light that test light bulb. Try switching to a digital voltmeter. That should be able to find it.
To fix it, pull a fuse at a time until you find the right circuit (volt meter no longer shows a drain when the fuse is pulled). Once the circuit is identified, time to do some detective work. Be patient! Some circuits can be pretty big and will take some serious time to figure out....
In the end, knowing you found the issue and fixed it on your own is worth giving yourself a pat on the back!