An automotive radiator IS a battery with the coolant acting as the electrolyte. The acidity of the coolant is reflected in the radiator's ability to store current just like the lead-acid battery used by the electrical system. I hadn't seen the voltage values listed but certainly can't disagree with them. I've always looked for very low voltage values and have actually seen a couple of systems that would light a 12V test light (not common).

If you're getting a bit of voltage that dissipates in a short time I'm inclined to think that the radiator isn't grounded (most ground thru the attachment bolts) so it's acting like a capacitor and temporarily holding power though I'll have to think about this a bit more (scratching head.....).

Dan