The Missing Heirloom

Detective Jones was called to investigate a puzzling case. Mrs. Wilkins, a wealthy widow, reported that her family heirloom, a valuable diamond necklace, had gone missing from her locked safe. The safe showed no signs of forced entry, and only a few trusted individuals had access to the combination.
As Detective Jones arrived at the scene, he carefully examined the safe, the room, and interviewed the people who knew about the heirloom. Here are the clues he gathered:
Mrs. Wilkins last saw the necklace a week ago when she checked her safe. She always kept it locked when not wearing it.
The maid, Sarah, had been working for Mrs. Wilkins for over a year and had occasional access to the safe to clean around it.
Mrs. Wilkins' son, John, was visiting from college during the time of the theft. He claimed to have no knowledge of the necklace's whereabouts.
The family lawyer, Mr. Thompson, had recently visited Mrs. Wilkins to discuss her estate planning. He knew about the necklace and its value.
There were no signs of a break-in or tampering with the safe's lock.
After careful consideration, Detective Jones solved the case and found the missing necklace. Can you figure out who took it and how?

Solution:
The thief was the family lawyer, Mr. Thompson. Here's how Detective Jones deduced it:
Mrs. Wilkins always kept her safe locked, and there were no signs of forced entry. This ruled out an outsider's involvement.
Sarah, the maid, had no motive or opportunity to steal the necklace. She had been working for Mrs. Wilkins for a long time and had never shown any dishonesty.
John, Mrs. Wilkins' son, was visiting and had no reason to steal from his own family.
Mr. Thompson knew about the necklace's value and had recently visited Mrs. Wilkins. This gave him both the knowledge and opportunity to steal the heirloom.
The lack of forced entry indicated that someone with access to the safe had taken the necklace.