The Duchess's sapphire and diamond sparkler seemed to mysteriously vanish
Credit: PA Photo/Landov

When the Duchess of Cambridge and Prince William arrived for a special visit to a cancer hospital outside London, her world-famous engagement ring was there for all to see.

But more than two hours later, as she left the Royal Marsden’s new $28 million unit for children, some observers noted that the sapphire and diamond sparkler (previously owned by Princess Diana) had mysteriously vanished.

A palace source tells PEOPLE that Kate, 29, had slipped off and placed it in her clutch – for hygiene reasons on advice of the hospital staff. She stored it in her bag and carried it out in that afterward.

“The plan was always to take it off when she went into the ward in the same way the Duke and palace staff rolled up sleeves and tucked ties into their shirts,” the source says.

The bag, and William’s jacket, were left in the care of hospital staff while they toured the Oak Centre, which the couple formally opened.

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Indeed, when the royal couple met two young leukemia sufferers in the new unit during their two-hour visit, William walked in with his suit jacket off, sleeves rolled up and was rubbing an antiseptic gel into his hands.

The visit to the Royal Marsden, of which William is president, followed a private visit for the Duchess to the National Portrait Gallery. Insiders say that it is all part of her ongoing project to pick charities and causes that she could work with and also learn more about the major cultural bodies in the country. “We were delighted to have had her here with us,” spokesman Neil Evans tells PEOPLE of Wednesday’s “behind-the-scenes” tour.

Later Thursday, Kate helped William and his brother Prince Harry host a private reception at St James’s Palace to thank those who have been helping their joint charitable foundation this past year.

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