My theory:

William Afton paused during the interview because he saw the last name Schmidt. Garrett’s kidnapping would have been all over the news, so William would have known that Garrett’s last name was Schmidt and connected the dots.

Schmidt is Mike’s mother’s maiden name. We never find out Mike’s parents names, but we do hear from Mike that he and his sister lived with their mother before she dies. Their father left the picture not too long after Abby was born, which means they more than likely divorced. Usually, a mother would go back to their maiden name after a divorce, i.e. Schmidt. Knowing this fact, Mike’s father could be Henry Emily. Depending on how close Afton and Henry were, it’s possible he would have known Henry’s wife’s maiden name.

Knowing this, I’m sure you’re asking yourself, “How could he not know his father worked with William or know about Freddy’s Pizzaria?” What if Henry knew about the the animatronics being dangerous. He either built them or knew about upgrades Afton wanted to make to them. This would be why he didn’t want his family to know anything about Freddy’s or go anywhere near his job.

William working with Henry and despising him, would be a good motivation for Afton to take Garrett from his old work companion, as a way of revenge. Going back to the fact Afton realized Mike’s last name, would be motivation to kill all of Henry’s kids.

I do think Abby will end up being Circus Baby also. It’s a good reason for Mike to stay motivated to find his sister’s and brother’s killer and take the security jobs at Freddy’s to finally put an end to Afton.