[He doesn't particularly want to admit that he hasn't really got an idea how to reverse track a piece of equipment like that, so he just shakes his head.]

Lestrade will only make this official and possibly spook Moriarty into prematurely killing Molly, better that we deal with this ourselves. We can certainly scope out Caroline's place, but I'm nearly certain she won't be there. I know where she'll be, I just need to think and it'll come to me.

[He does know it, it'll be buried in whatever Moriarty said, but he doesn't know what he knows yet.]