Yeah, it's a bit weird, but everything that is hidden is visible when you are very far from it (mines, hidden chests, things you have to use your shovel on, switches...etc) and then become properly hidden when your characters approaches.
The mines on the beach are meant to be invisible, by the way, from what I've seen there is absolutely no mines which aren't hidden in the game, which makes sense since your character automatically avoid it when revealed...