My 2004 XJ8 kept giving me a COOLANT LOW message every two months or so, and each time I would look, and, yep, the coolant level was too low.
It was easy enough to add a little coolant when it did, but it seemed to me to happen too quickly.

I asked my mechanic to look at it, but he didn't notice any obvious problem.
When I got the car back, I looked myself, and thought: Damn! My mechanic didn't put the gasket back in the radiator cap!

I bought a new radiator cap on ebay for $8, because it was the easiest way to get a new gasket of the right size.
While taking the old one off, I found the gasket was still there--it was just so squished after 18 years that it was almost invisible.
I put the new cap on, and haven't had a COOLANT LOW since then.

 

Xeno, thanks for the tip. I have had the COOLANT LOW light come on and it's unnerving to be driving down the road and have that light come on.