Call off your searches
I found my retainers. How big of an idiot am I, you ask? A pretty big one. It was in the bag that I had planned on putting it in, the one I thought it would be in, but apparently even though the bag is about 2 inches by 3 inches by 6 inches, I couldn’t see that the red case was in there until I emptied the bag, which I was doing today as I was organizing things. So goodbye gap and hello pain for the next few days.