I have wondered about something with the challenge coin thing. I wasn't in the military, however I have one what I term real challenge coin. My brother sent it to me when he made Command Master Chief, it is his coin. He also sent one to my kid, and my nephews. I do often carry it with me. However I would feel strange presenting it during one of these checks, since I wasn't in the military.I was doing the challenge/coin check. It works much better sitting at the bar than on the INGO.
If you've been presented a coin, you have to carry it. (Not really realistic, I carry mine from the RoK pictured above since it was the first coin I was presented.) If someone challenges (taps coin on bar,) everyone who has been presented a coin (there is no database of everyone presented coins) responds by tapping their coin on the bar. Everyone has a coin? The challenger buys a round for the bar. Someone doesn't have their coin? They buy the round.
My last few months in Iceland in 1998 (Sheryl and the kids had already moved to NC) I lived in across the hall from the Zoomie Bar. Always had my coin, got a lot of free drinks.