Long after her affair with former cavalry officer James Hewitt was over, Princess Diana kept his signet ring locked in a box that's now missing from her estate, a police officer testified Monday in the trial of her butler, Paul Burrell, according to the Associated Press.

Detective Sgt. Roger Milburn said in court that, at the request of Diana's sister, Lady Sarah McCorquodale, officers searched for the box at the home of Burrell, 44, but turned up nothing, the Guardian newspaper reports.

Burrell has pleaded not guilty to three charges relating to the theft of hundreds of items from Diana and other members of the Royal Family. The trial began a week ago Monday but was suspended briefly on Wednesday, when the original jury was dismissed. A new jury was picked and the trial resumed late in the week.

AP reports that the detail about Diana keeping a ring from her former lover was deemed so sensitive by police that Milburn had refrained from saying it aloud in the courtroom last Friday. He instead wrote the information on a piece of paper and then handed it to the judge.

The jurist later decided it could be disclosed. (Hewitt, deemed a national heel for talking about his affair with Diana, also published a book about his fling with her.)

The ring from Hewitt was apparently found in a locked box kept in Diana's living room and unlocked after her death by Burrell and Lady Sarah once the key was found hidden in a tennis racket cover.

The box also contained letters to the princess from Prince Philip, audio tapes from an unidentified former employee and the resignation letter of her former private secretary, Patrick Jephson -- but the box reportedly has been missing ever since.