[folivier]

Tom, you will have some air bubbles in the cooling system which will not compress at the same time. I did quite a bit of water testing (at 2150 psi) and in almost all cases we always had air which would have to be compressed. So a 2 psi drop overnight is to be expected. And temperature has a definate relationship to pressure.
I would think the 2 psi is just air bubbles. If it was a leak it would continue to drop, either slowly or rapidly.