I just want to point out my mistake in my last post, I got to rewatch the movie Saturday and while it's the belt buckle with the name Botany Bay that makes Chekov remember, he does shout that "On Ceti Alpha 5 there was life!" at Khan, so he did know about the planet Khan was sent to. It must have been just bad memory, I'm sure he's been to a lot of planets.
I do wonder why only one scanner showed "something" ? Shouldn't have showed 50 some humanoids?