[@BCTheEntity] I'll say it one more time;
Psychic energy doesn't matter. Telepathy matters.

[@Sophrus] No, they wouldn't be. Any psyker with a brain knows better than to waist the time. If they wanted to pull off a covert attack, they would cloak their presence in the warp. They're eldar, they know they glow psyker, and they're still considered masters of stealth and surprise. Piss-ant astropaths wouldn't be able to find them if they didn't want to be found.
If the eldar weren't trying to be sneaky about getting on board, then you wouldn't need an astropath to tell you they had. You'd be able to tell by the giant wraith ship that just rammed your hull.

We were protected by the fact that no one should be crazy enough to try and pull off what we are. However, we're protagonists, so the plot is on our side. The plot is still on our side, we'll just have a harder fight of it because they're going to spot the foreign psyker. No, they're not going to respond with full-scale counter boarding action. More likely, they'll suspect one of the penal legion has somehow been corrupted by chaos. Criminals turning out to be cultists does just happen.