Twice in a row I've found on customer car that their radio fuse is the culprit.
1. Use a multimeter, set it to measure amperage.
2. Disconnect the negative battery terminal from the negative battery post.
3. Put one probe on the negative terminal, put the other probe on the negative battery post.
Now everything that make an electrical loop back to the negative battery terminal will have to go thru the multimeter and you'll be able to measure the current.
4. With car off, key out, the amp value should be very very low in the .01A range, anything more than that then you have an electrical draw somewhere.
5. Start pulling fuse until you find the electrical branch responsible for your draw.
Don't forget that the modern day car alarm and other car electronic accesories sometimes is still connected to the battery even though it is not on and could be responsible for your slow electrical draw as well so if you already exhausted all the factory fuse points, start going after all the aftermarket electronic add ons.