François Ruffin had just started filming a satirical documentary about Bernard Arnault, France’s richest man and head of the LVMH Moët Hennessy Louis Vuitton luxury conglomerate, when he became suspicious about a volunteer on his staff.

The man, who went by the name Marc Foll, had been spotted digging through the trash. He leafed through papers on desks. He carried a camera pen and made conspiratorial remarks. Staff members began referring to him as “the mole.”

In the film, Mr. Ruffin, 49, a gadfly journalist, sought to confront Mr. Arnault over decisions he had made, including outsourcing jobs and closing factories, that left thousands of former LVMH employees struggling. It was only years later that Mr. Ruffin confirmed his hunch — Mr. Foll was in fact a spy whose reports made their way back to LVMH.

Details of the surveillance were exposed during an influence peddling trial last month concerning France’s former intelligence

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