Finally resolved. It took me going there in person as no one was getting back to me or answering the phone.
Service writer who let her drive off with no cap was there and gave the "I tolD heR iT woUld bE FinE bEcaUse it iSn't PreSSuriZEd". I brought the owner's manual in hand and had him read the section to me that states otherwise. He walked back to the shop, had a conversation with a tech and then apologized to me saying he was given false info from the tech. I told him fair enough, please grab me a cap and a bottle of coolant and I will be on my way.
I am going to seek another Hyundai dealership to do the maintenance with. I didn't like the vibe when she bought the car there from sales, I didn't like the vibe when I dropped it off for service once in the past and now this. Crummy dealership all around.
The service manager who never answered my calls or return them was there and didn't seem phased by any of this.