Back in Casey’s room, Ed and Casey gazed at her laptop. They had raced through the hotel lobby, the night porter looking up blearily, surprised by the sudden movements.

‘Come on,’ Casey grumbled to herself, searching frantically. ‘Come on.’

Casey had tracked ships around the world before. Almost all large ships sent out a signal, and on her computer she had trailed the colourful little dots for days at a time, as they swept across the map. The shipping lanes lit up like motorways: ships hurrying around the Cape of Good Hope, cornering around Cape Horn and shouldering up through the Strait of Hormuz. They clustered at the bottlenecks – at Panama, at Suez, at Malacca – queuing up impatiently, every minute costing.

In the past, as she worked, Casey had watched the pirated ships, drifting aimlessly off the coast of Somalia. Still sending back their hopeful signals, we are here, we are here. Even though no one would ever go to their rescue.

Once, Casey had hunted down a notorious billionaire, glorying on his superyacht, and he had lashed out at her photographer, in the pretty harbour of Cephalonia. How the fuck did you find me?

The Post’s business desk routinely tracked a few of the supertankers, because the giant ships unwittingly telegraphed the traders’ expectations about the price of oil. Sometimes, the vast tankers would be filled with oil, enough to fill a hundred huge swimming pools. But instead of beetling off busily, in their dead-straight lines towards Europe, and America and Australia, the ship’s huge engines would be switched off, drifting with the current out to sea. And there they dawdled, as the traders waited for the price of oil to rise. A strong vote of confidence, that was, with the tankers costing $50,000 a day just to leave out in the ocean.

Ships could switch off their trackers though. Casey had watched ships hovering in the Mediterranean, just off the coast of Syria. One second to the next, they would melt off the screen. Vanishing for a few hours or a few days before they reappeared, all innocence, heading back up the Bosporus towards the Black Sea.

Now Casey focused on the fleet of dots gathered around the port of Chittagong. The huge cargo ships came and went in their hundreds from Bangladesh. Whisking away the cheap clothes, the cheap food, the cheap goods.

She checked the ships, one by one. The Kristina. The MT Scorpio. The Somerset Lady. The Dolphin Blue. And at last she saw it.

‘There,’ she breathed. ‘The Tephi. That is what they were talking about.’

Ed and Casey stared at the tiny dot. The Tephi’s beacon had sent its last plaintive message close to the coastline by Bhatiari, 10 miles to the north of Chittagong.

‘That’s lucky,’ breathed Casey. ‘I thought they might have switched off her tracker earlier. They often do.’