electrical leak?..

Remove the battery lead and install an ammeter inline for measuring "car off" power draw.

Then pull fuses, one at a time, to see if the excess remains.
When you pull a fuse that kills the excess, that's the circuit that's drawing too much.

If none of the fuses kills the excess, then it's an aftermarket circuit connected "upstream" of the fuses, and you'll have to trace wiring from the battery.

Any additional wires connected to the battery "plus" terminal?


Of course, if you've had this car a while and this is a new symptom, there's a simpler approach:
What did you have installed just before the problem started?
E.g.: A stereo with an external amp? A radar detector? A recovery system?

Over 20+ years, cars tend to pick up a lot of "extras".
A couple of years back, I dropped my '88 at the dealer and told them to rip out anything that wasn't on the factory wiring diagram.
It took 10+ hours of labor, and I got back a big box of extra parts. (Remote kill, radar detector, ext amps, and stuff I can't identify.)
But the Interstate will sit all winter without being drained, now.

 

There is a good chance that the alternator is bad. If DGS suggestions come up blank, have a hard look at the charging circuit. As shown below, the battery is connected to the fuse/relay panel and also to the starter, and from there to the alternator. Bad diodes in the regulator can drain the battery.
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If you have a "leak" you will usually see smoke. That's the way us old Jaguar guys used to find them. Just had to say that.