If you say the plastic is getting damp maybe try taking the shroud off and looking at it (it is a pain in the ass to remove but if you can't find where it's coming from you may have no choice). Even if your coolant "evaporates" on the radiator, it will leave behind a dried trail so you can follow it. Warm it up, park it on ramps if you have them and then see what you've got going on under there. Even with all aluminum, it is possible to have a pinhole develop.