[@Klomster] Actually, every warp-capable ship in the Imperium has its own, individual navigator, if not more. And astropaths, while not all especially powerful, are all trained and sanctioned psykers. They would have far more experience than Lucius's toady, and as you kindly pointed out, there would be an entire choir of them. Given they would all be actively looking for telepathic signals, they'd be almost guaranteed to spot him. And, yes, they can tell specifically if he's another astropath that's doing it, so they know he's not imperial.

The problem isn't that he's a psyker. The problem is he's using telepathy on an imperial vessel, and has no idea what he's looking for, or how to do it so he doesn't get caught. Because astropaths undergo a ritual that allows them to sink up enough that they can communicate between literally any two points in the imperium, they know their own from random amateurs. Given this is the imperium, they'll know if any other telepaths are supposed to be on the boat, and will mark him as an unsanctioned psyker. And, everyone knows what the imperium does to unsanctioned psykers that have the smell of chaos to them...

Poor lil' dude's gonna catch a bolter shell to the head, and that's if he's lucky. If they capture him and an ordo hereticus catches wind of him, he'll only wish that was the case.