Wellll Michiru can change forms at will right? She totally noticed something was a bit.. Different about Shirou and tested it out! Turns out she was right mwahaha
NOTE: I had posted this earlier today and took it down because someone warned me that Michiru is 17. However, after some research, I discovered that as of Episode 4 of the anime, she is 18 (she has her 18th birthday literally in the episode). So in the current canon of the story, she's an adult. Even so, I'll be more careful about verifying characters' ages in the future if there's ever any doubt. This one was a close call, but is safe!