Don't know how to comment on your last sentence?
But surprised you only found one Corona bottle cap, as it is favored at the beaches where I treasure hunt.
We have lots of rainbows, but I have yet to find that unopened case/ six pack (local gold) under one. An added plus as far as their bottle caps sound the best of all I have detected (discrimination mode) on my Xcal, especially the fresh drop(s). Sanding not a problem as they tend to float.
You just never know.....I'm sure I have a couple of old boys on the beach tossing coins in the water now when they see me coming. I always give them the sinkers I find so I think it's their way of paying me back. I've tried to talk them into throwing their wives jewelry out there but they told me that would cost me a lot more sinkers. Let me tell you, these two guys are a couple of real enjoyable cards.