Xeph doesn't blame the imperium for gunning him down. He was on the wrong side of the war at caliban.
Also, xeph might do some surprising things in the near future.
Also, xeph might do some surprising things in the near future.
There's a difference between using a psychic attack, which just uses warp energy, and sending out a telepathic homing beacon. Yes, a psyker can notice another psyker using warp energy. However, it's generally not something you can do through multiple layers of hull without actively looking for it. Chances are, the two head psykers on board would be so focused on the battle they're in they wouldn't just be randomly scanning the ship for other warp users, seemingly without cause.
But, a foreign, chaos-tainted, and far less experienced psyker actively scanning the minds of others on the crew over an extended period of time? Astropaths are masters of telepathy, and they would be actively looking for telepathic assault from enemy agents. That is, almost literally, their job description.
So, yea, far more likely the glowing beacon is going to get picked up on enemy radar than the single, random blip among a background of bigger blips.
The astropath would pick up the psychic ping, yes. But apart from sending a message to the captain. (As in radio or a runner.) The astropath probably will not alert many of the coming problem.
Since a psychic beacon/sonar can be created for many reasons. Perhaps a new psyker awakened? The ship will not go on full alert because of that.
But they probably will send a team of armsmen for it. To check it out.