Middleton, 31, accidentally revealed a candid picture while leaving London on July 24 when a fellow traveler passed her on the escalator at Heathrow Airport.
"She said thank you and was extremely nice, and then I did a double-take and instantly knew who it was," Grace Stroup, 16, tells PEOPLE.
Standing behind Princess Kate's sister, the royal fan had the perfect viewpoint. "She pulled out her phone and her lock screen had a picture of George and Kate, smiling together looking just like a regular family – it was so cute!"
Traveling solo, Middleton, who was wearing blue jeans with ankle boots, "seemed in a rush to get to her gate" but was "extremely lovely and polite." Meanwhile, Stroup was so excited, she took to Twitter to share her news, calling the marathoner "gorgeous."
"She had a grace about her," says the high-school student, who couldn't wait to text her mom and her brother too. Still, "it was surreal to see her looking so normal."
See Prince George Show off His Toothy Smile in Adorable New Birthday Portrait
Well, aside from that unofficial, never-before-seen picture of the future King of England on her phone!
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