Dream like a short movie

This dream was weird in that it didn't involve me in any way, but was like watching things happen to someone else, and actually played out like a story (like seeing a short film, hence the thread title).

I dreamt about a man, an old man, who worked at a dockyard in a vast hall where big curtains of tarpaulin were stitched together to line repair docks for ships. (This felt like a setting a few hundred years ago, but I'm not sure exactly when.) He would go up big ladders to attach sheets of tarp, high above the ground, day after day, but he was doing something else too. When no one was looking he would sew in pockets, where smuggled goods could be hidden, so that once the tarp was submerged the smugglers who were paying him could hide and retrieve their stuff. One day though he was seen, and although nothing happened straight away, he knew that some special punishment was coming his way. And sure enough, a few days later when the latest tarp was being lowered into the water, he was grabbed and thrown in. He couldn't swim, and the people who'd thrown him knew it, but he'd already made his plans. He thrashed along the side of the dock, looking as if he was struggling for his life, but really searching for a mark he'd made on the waterline portion of the material, then he took a deep breath and allowed himself to sink. (In the dream he actually had a parchment map in one hand, helping him to locate the right spot, and he could hear people on the dockside wondering why he was counting off the yards as he pulled himself along.) Ten, twenty feet he sank, before getting to a flap of tarp just big enough to squeeze through into a pocket large enough to hold both him, and the air he'd known it would trap when it was lowered into place. There he planned to wait until nightfall, before making his escape from those who now thought him drowned. (The dream ended at that point, so I can only presume he got away okay)
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None of the people, places or situations in this dream are in any way familiar to me. Any ideas where it might have come from?