“There’s one man in the wheelhouse and another on the bow, as if he’s a lookout, and he’s holding something,” she said, passing Bruno the binoculars. “It’s as if he’s doing an inspection. You’d think they’d be hauling in as many oysters as they could for the New Year’s market.”

Bruno found it hard to focus, but she was right. The boat was behaving differently, and the deck seemed clear of oysters. All the other boats were piling them aboard as quickly as they could. “I can’t see what it is he’s holding, but there’s something in his hand. It’s as if he’s looking at his phone to check something on the Internet, but can he have a connection out there in the bay?”

He searched for a name on the boat but saw only a number, hard to discern.

“I can make out the capitals A and C and then three-two-two, but there are more numbers I can’t distinguish. Can you try?” He handed her the binoculars.

“I get the A and C, and I think there are two nines at the end. The one in the middle could be a seven or a two, even a four or a six. It’s faded,” she said.

Bruno fumbled in an inside pocket for his notebook and pen and scribbled down the numbers he was sure of and the ones Isabelle had guessed at. When she asked why, he explained about the fake oysters and the sensors they had all been issued.

“It may be one of our guys checking that the transmitters are working, but if not, we could have a problem. We’d better head back, and I should call this in.”

He pulled out his own phone and called the number for the operations room, gave his name, and described what he’d seen and asked if anyone was checking on the sensors and transmitters. He was told to hang up and stand by, they’d get back to him. Bruno and Isabelle managed to scramble down the dune without tripping, and as they walked along the beach, she asked what time he had to report for duty.

“Six,” he said as his phone rang. It was a deputy commissioner. When Bruno had repeated what he’d seen, he was asked to report at once to the Arcachon police station. He explained where he was, and they sent a car. Bruno suggested they should drop Isabelle at the hotel.

“Are you trying to protect my reputation?” she said, with that impish grin he adored.

“Given what you said about Commissaire Pleven…” he said haltingly.

“Bruno,” she said firmly. “The day I’m not prepared to be proud of the man I’m sleeping with is the day I enter a nunnery. Besides, Pleven and I are of equal rank.”