Welcome Shannon. Your profile indicates you have a '99 S-Type.
Battery drain - first question is how old is the battery? They don’t last forever.
Is it dirty and corroded? Dirt conducts electricity and corrosion impedes conduction, so be sure your battery is clean.
If not that, then something else is not shutting off, could be a relay.
There was a transit isolation relay in the trunk (UK = boot) that the first dealer should have removed, and tucked the wires for it up away from shorting contact. Is there water in your trunk? These wires have been known to fall down and get in the water and crud that builds up in the trunk.
Here is a link to an old thread on this subject. Click on the title to read it with the responses.