[@Klomster]

Quote: "[b]Every starship[/b] that plies the Warp routes of the Imperium and beyond [b]has at least one[/b], and most likely an entire cabal of Navigators to guide it safely through the Immaterium."

[url=http://warhammer40k.wikia.com/wiki/Navigator#Resources_of_the_Navis_Nobilite]Emphasis mine[/url]

Also of note, the [url=http://warhammer40k.wikia.com/wiki/Astropaths#Astropath_Shipboard_Actions]list of powers[/url] that astropath choirs can pull off are amazing feats. When working together, they are not to be underestimated.

[@BCTheEntity] Again, the problem isn't that he used a psyker power. The ship is in combat, against eldar. The astropaths aren't going to be scanning the ship for psykers. Any psyker just being on board, without even casting a spell, could be spotted by an astropath looking for one if the psyker doesn't know how to mask himself. What's hiding us is that they have no reason to be looking, or they didn't.
The problem is that he's using telepathy. The core purpose of astropaths is to look for telepathic signals and powers at all times. They spend years in training to do exactly that, and then gain more years of experience doing it every day for the rest of their lives.
They Will identify him as not being another astropath. Astropaths all undergo the 'soul binding' ritual, that leaves them completely purified. That's how they identify each other in the warp; because nothing else in the warp reads like they do.
Then, once he's been spotted, the astropath will turn his attention to finding him, and do so within minutes.

So, yea... pretty boned.