Maybe Flynn's patter was working, maybe he was just curious. Whatever it was, the guard came closer.

Flynn made a show of looking around. "I only want to tell you," he said, lowering his voice, "because you look like someone I can trust. But I can't tell if anyone's coming."

He waited to see if the guard took the bait. Even if he just locked himself in the cell with them, that would be good enough to get to the key.