Thanks guys, the cap is 5 months old, purchased at the dealer. It's one of the new type.
Doesn't mean it's not faulty though.
Temp never went above 180 until the red low coolant light started flashing then it started creeping up a bit. So I don't think it was going out the overflow tube or anything.
It's at the Indy shop right now, because I didn't want to have it towed home just to tow it again to the shop if it was over my head. I'll have him check the cap first. Trunk appeared dry, but it hadn't lost a lot of coolant yet.