"I know" Ahahahahahahahahahahahahaha!
My (now 16 year old) daughter did that too! Except she was only three so it was still cute. She has pretty strawberry blonde hair and people would always comment on it. They still do, but not so much now that she is a 'scary' teenager in a Metallica hoodie. Anyhow, I guess she was so used to hearing it.. Random lady said " You have very pretty hair" My daughter
literally flipped her hair back and sighed "I know". At
3! My response was similar to PennyPinchingPixie and we got a reminder about manners, but the lady was laughing and said something about how she must hear it all the time.
She sure did. She went through a phase a few years later where whenever she would catch sight of her reflection she would say "hello beautiful" to it.

even in a mall window! (We still tease her about that!)
I haven't noticed any "autopilot" responses yet..... I'm worried!