(SPOILER)I thought Sylar did hear them and was playing dumb. Later, in the car, Sylar mentions Noah's plot when he's trying to persuade Claire about her Dad. Noah has a look of, "Oh crap, you heard that?" They never spoke of the situation in front of Sylar so either: A) Sylar heard what was going on and played dumb in case he could use it to his advantage or B) Claire and Noah mentioned something about it in front of Sylar between the carousel and when we cut to them in the car which we didn't see. I still think it's A.