I know your post was years ago but I’m having a similar issue with my XK8. It’s a 2000. Don’t drive it for a few days and the battery is drained. Something has to be running. No way the battery should drain that fast. Thanks!

 

First make sure that your battery is on good condition, after that you would usually pull fuses one by one and check for current in the sockets, only keep in mind that car is in some semi stand by for some minutes after locking and will draw a bit of current during this time

 

Look, first, to a light that stays on in the glove box; ditto, boot. Use your smart phone in video mode, close the doors. . . .

Then start searching for parasitic draw relay circuit by relay circuit one at a time: lots of threads on this forum on how to do it; also YouTubes.

 

Jaguars do NOT 'shut-down' immediately when key-off, doors closed.

It will take 20 minutes to 45 minutes depending on the model and year.

The 4.0 X100/X308 should 'go-to-sleep' after 20 minutes or so.
If you watch the battery draw with a current meter, it will gradually use less and less power over time as systems shut down one-by-one.

It is painstaking and annoying to wait while the car shuts down and pulling/reinserting some fuses or opening boot lid or doors will 'wake' the car up and the procedure needs to start all over again.

I open the boot lid and snap the latch CLOSED. I open ALL DOORS and trigger all the latches CLOSED.
NOW JUST WAIT and WATCH.

The final draw should be about 30ma + or - a few.

 

It's very time consuming to check all the circuits with the delayed shutdown going on. But having done it, I found my larger current drains were related to seat and window controllers. However, I only resolved the problem for good when I changed out the security locking module. If I remember correctly, it didn't have the highest draw, but I believe this module tells the other systems to power down when the car is locked. YMMV but with hindsight I'd start there instead of chasing the highest draw but definitely check the stuff mentioned above.

I paid about $60 for the replacement SLC module and it's best to try to match part numbers exactly to avoid problems. In particular the key fob remote frequencies changed at some point.

I had a 3-day battery life before swapping the SLC and I've since gone as much as 4 weeks between starts without problems.

 

4 weeks between starts must be its own kind of painful! The only time mine goes that long is when we’re out of the country.