On a cold February night, David López Ibáñez set out to have a normal evening.

Mr. López Ibáñez, 31, went to rehearsal at the London-based Philharmonia orchestra, of which he is a member. Then, he and a friend went to a pub near his new apartment for a drink and a bite to eat. He took his violin off his back and leaned it against a bench. It was there as he ate, just by his side.

Then, without his noticing, a person grabbed the violin case, hid it under a long, beige coat and walked out into the night.

“I stood up in disbelief,” Mr. López Ibáñez said in an interview, recounting the moment he realized the instrument was gone, adding, “It was a companion. It had been on my back and I had been playing music on it for eight years, all over the world.”

For a serious musician, an instrument is more than just an object. And this was not just any violin. It was made in 1740 by Lorenzo Carcassi, a master craftsman, in Florence, Italy, and was valued at more than 150,000 pounds, or about $200,000.

“You let your guard down for a second,” Mr. López Ibáñez said, recounting the loss and the scale of what happened. “It felt like the world was collapsing on top of me.”