My guess is that you were leaking coolant on start-up... and maybe before that.

In CA, I bet this car has seen some traffic... I would check to see if both fans are working..Do the newspaper test on the mech fan... The fans do fail and will not generally give a code.

I have done a head gasket on my '05 3.0...It's easy...even for a first-timer. We can talk you through it.

I rent the time-sert tools.

Up to you whether to buy another one..They are pretty cheap...But if your X5 is in good shape and you like it, I would fix it.

You can try and put more coolant in it and see if it holds, but I think the head has to come off... it will give you a chance to replace coolant pipes and a few other things.. Compression test might not tell you much...I had coolant pooling in cylinder 6 yet I had good compression on all 6 cylinders. I think swabbing the cylinders for coolant the morning after running it to op temp is a better test.

Blocks can and do warp, but the likely issue in your case is that the head is warped and the block is okay...

Dive in, man...