Dressed in a baby blue coat, Kate, who will turn 32 on Thursday, checked out the home furnishings and picked up some supplies at one of her favorite department stores, Peter Jones, in London's Sloane Square.
Also spotted in the store: her mother, Carole Middleton, 58.
Fellow shopper Louisa Smith saw Kate in the cashier section, and noted that there was very little fuss about the VIP.
"You wouldn't have known it was Kate Middleton," Smith, an independent distributor for a health and wellness products firm, told PEOPLE. "The shop is always quite calm anyway, and there wasn't a great deal of people around."
In fact, said Smith, "It was only when I did a double-take that I realized it was her."
Once the recognition factor kicked in, Smith was very impressed: "She was absolutely beautiful. She has a very magnetic energy about her. She looked absolutely stunning."
Besides, said the shopper, "It was nice to see her doing ordinary stuff!"
As for her birthday, it doesn't look as if Kate will be able to spend much of it with William, given that he is commuting 60 miles north to Cambridge from their home for the day. (As royal insiders point out, it's a bit early in the term for him to start cutting classes.)
Still, if his first day back to school was any indication, the two should be together again by evening.