Although Chekov could find it with a mere tricorder.It was clearly shielded from sensors
"Captain, five hundred metres in that direction there's a foundation under the surface. A huge one. Registrations all over the place... Immensely old, completely buried."
It's a silly episode. ("Ion power!") Enjoy it for that, but don't try to inject any logic into it that isn't there. Some dislike "A Piece of the Action," but I enjoy that one for a tongue-in-cheek allegory about the consequences of what later became the Prime Directive. It's like International Talk Like A Pirate Day. Imagine the fun they could have with that one: walking the Planck with the locals "parroting" the Federation, matey.



