Well, the detective who said he didn't notice was not the dog handler and wasn't there to observe the dog handlers. The defense elicited the "didn't notice" statement, I believe, perhaps to throw doubt on the ability of dogs?
Then on cross-exam Dusek brought out the clarification.
Though it is odd that the detective wouldn't just state "I don't know" instead of "I didn't notice". Using the word "notice" does leave an impression that if there had been some kind of reaction he would have taken note of it. At least that's how the phrase plays to my ears.