[Shiny]

How did you discover this? How long does it take to drain your battery after charging it? Basically, how much current is flowing? A little or a lot?

Regardless, I don't think there's an easy way to guess at what is drawing current.

If it were me, I would first try to isolate the circuit that is drawing power.

I would do this by systematically pulling fuses while monitoring current. It may be possible to isolate your problem with voltage readings but that's complicated and access to "test points" is a bear. Current measurement will be more direct and easier IMO.

The only risk is if the current draw is high enough to damage your meter, but your description makes this seem unlikely.

What is the maximum current setting on your meter?

If you cannot ballpark the current level and you are worried about damaging your meter, find a fuse that is the same amp rating as your meter's maximum current reading. Disconnect the positive cable from the battery.
Set one end of the fuse on the positive battery terminal and touch the positive cable terminal to the other side of the fuse. If it doesn't blow, you have gained confidence you won't damage your meter.

Then put your meter leads between the positive terminal of the cable and the positive terminal of the battery. You are completing the circuit with your meter, which is the way to measure current.

At this point, you should read a current if your load is always present. If not, the challenge becomes bigger because you are dealing with an intermittent or "situational" draw.

Assuming you can read a current, start pulling fuses until you find the circuit that is drawing power. You can replace them immediately if you don't see the current go away and move on to the next.

If you go through all the fused circuits and don't see the current drop, you have still made progress, as you've eliminated a LOT and can start following the few wires and connectors between the battery terminal and your fuse block, starter, etc.

Mike