They knew they were heading somewhere to work, through lies and deceit since obviously the trains went to Poland. It just wasn't their assumption they'd be getting killed there. Also given the camps were prisons themselves, pretty obvious they'd know what the chimneys were for. They'd see the kitchen near one crematoria and see the smaller chimneys. I don't think they'd believe they would keep a shower area under a kitchen building, especially with a single huge chimney.
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Also, people tried to escape Auschwitz, too. So it's safe to assume a revolt can't be ruled out.















