I have suffered battery drain in my S-Type (facelift) for over a year.
Recently, replaced the lights ECU above the fuse box and front electronic module. Thought this would help and the dead battery that happened once I put down to it being drained to 0 so many times that it is due replacement.
However, yesterday I was in work car park and noticed the HIDs coming on for a few seconds again.
What could cause it?

 

There are detailed threads about this with helpful info so have a Search and read. You'll need a good meter and to be persistent, logical, thorough, etc as these faults can be tough.

 

From my experience, it is likely your Multi-Function-Switch. However, there is also an excellent battery drain thread in the stickies that you may need to methodically work your way through. MFS is not a cheap part, so not one you want to replace haphazardly.
MFS:
https://www.jaguarforums.com/forum/s...solved-175596/
Battery Drain:
https://www.jaguarforums.com/forum/s...-drain-123535/