Hi. I have some kind of electrical issue with my Jaguar. At first I thought it was a flat battery, but after charging strange things began to happen (before putting it back in the car battery had almost 12V, I checked it with multimeter). When I connected it to the car, I started to hear a strange clicking noises, lamp in the trunk lit up for a moment and then it goes out. The car is a Jaguar XF 2009 3.0 diesel. Any ideas?

 

12v is not enough to run your car, minium of 12.3v.
A fully charged battery should read around 13.2v

 

Yes. Your battery is dead. 12v is almost discharged. When the battery is too low on the XF something in the boot/trunk clicks and clicks until the battery is totally dead
Remove and fully charge it for 24+ hours on a good charger. If it doesn't get up to about 12.7V (differs a bit depending on whether it's an AGM or traditional battery - easy to find the tables online to show what you should see for fully charged voltage.

If that doesn't work, you need a new battery.