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 chief, Bernard Squarcini, whom LVMH had hired for security work after he left the post in 2012. Following a yearslong effort by Mr. Ruffin to press Mr. Arnault into contrition over lost jobs, the trial finally brought them into direct confrontation.

The case attracted attention with its revelations. A shark and a mole. Fake names. A homeless spy.

Mr. Arnault, 75, appeared in the packed Paris courtroom wearing a dark Dior suit and escorted by heavy security. For the famously publicity shy corporate titan, it was one of his rare appearances outside of his front-row seats at an LVMH fashion show.