If you're talking about the thingy to the right of the "6" in your picture, mine has what looks like a non removeable cover over it, and it is totally dry. Nothing, like your little "star cap" is exposed. You may want to go to the stealer and look at their illustrated parts breakdown and check if that is a separate item that you can purchase. Otherwise, that plastic "T" housing that the radiator cap sits on is part of the radiator. Better hope you don't have to replace the whole thing.....$$$$$$$.
Edit: This is real interesting. I just did a quick look at the FSM and, of course, I couldn't find anything regarding this part. So, I don't know what it's for, why it's there, what it does, or ??? If it doesn't appear to be a critical component (if it was I am sure they would talk about it), maybe you could just figure out a way to secure a cap onto it. You may also want to look around, maybe in the belly pan to see if it fell in there or onto some of the suspension components. I am going to go back out to mine and study it a little more.
06 GCO, Dk Khaki, QDII, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Debadged, Fumoto Drain Valve, BT Catch Can, Chopped EGR Tube, SRT8 CAI, K&N Filter, Sonnax Trans Line Press Booster, SC Tuner W/Adv Trans Tune App, DS Tuner, Cust tubular hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvr, SRT8 frt/rear bumpers, FRI Sidewinder Cam/6.1 valvetrain, SLP 25% UD Pulley, BBK true 90mm T-Body, SOS Ign Coils w/.060 spark plug gaps, SRT Reps with 265/45&295/40X20's.