Grogu immediately sensed Luke when Luke first got near the ship he was on...in all, I think guido is right here - this is one of those situation we should chalk up up to simply "The Force," and try not to think too hard about it. Luke knew where Grogu was because "Force." And we aren't just talking about any ol' force user here, it's the son of the Chosen One, whose force powers are naturally greater than many others'.