Update: 6 days later after I performed my flush I got a coolant low service light. Great. I open the hood to find the Expansion tank vent screw leaking in spurts and a pool of coolant in the under panel.
What do I do? Remove the screw, inspect it and hmm...looks ok.. screw it back on and lo and behold, the top broke off. Being a sunday morning, 1 am to be exact, and far from home, I ended up having to make an improv gasket "top" to seal it up and drive to the stealership where a $10 bill got me a new plug. (I bought two for backup)
I compared the gasket and the new one was definitely "fluffier" and not as flattened down as the old one.
Taking the rest of the screw out of the expansion tanks wasn't that easy but I finally got it.
Lessons learned, be careful when tightening that plastic vent screw. From now on, every time I do a flush I'll just go ahead and change that screw!