Tomorrow is Yesterday - S1-E20
Question: When Christopher is beamed back into his fighter jet, he doesn't see the Enterprise in the sky any more. The slingshot effect returned him to the instant before first viewing the Enterprise. He may not have any physical evidence to report now, but he would still have his "memory" of all the events that happened after he was first beamed aboard. If that's the case, he would now have to remain utterly silent about his adventures so as not to risk any change to the future birth of his son. Right?
Answer: He beamed back into himself, He jumped into a point in time before he was on the Enterprise. So When he didn't see the starship he had no memory because he never left.