In Star Wars: The Last Jedi the character DJ makes a deal with the First Order. He tells them the location of some resistance transports, so they can be shot down.
How does he know about these transports?
I thought the reason he is hired to turn off a tracker is because they don't know about the transports.
Because Poe Dameron told Finn and Rose over the comms while DJ was in the room, and he overheard. At this point, they thought he was on their side, or at least the thought of him betraying them hadn't crossed their mind, so they didn't think to not let him know about that information.
It took me several watches to find this, but DJ does overhear Poe telling the plan to Finn and Rose.
When Finn and Rose are on their way back with DJ, Poe contacts them and says "We're going to abandon ship!", (which he just learned on the bridge), "Did you find the Master Codebreaker?". At first, it isn't entirely clear that DJ is in hearing range, but there's a split-second shot of him looking up and paying attention.
