I could ask how the techno-shaman could detect them through the wall when they couldn't detect him (if nothing else, Psyniscience could have pinpointed his psychic ass from half the ship away) but leaving that aside for the actual rules discussion: 20 meters is hardly overwhelmingly far, and psychic powering his way through the wall is just gonna leave him with his turn ended and everyone's weapons pointing at him.
Heck, a Techno-Shaman's average damage on the Sledge is 12, so his ability to reliably get through anything stronger than cardboard is also kind of questionable.
Evidently he pushed his power to the max giving him Wp 120 and rolled 01 on his test for 6d10 +14 in damage. For an average damage of 47, but since he's a bad mofo, he maxed his damage.
His hidden prescence was due to a Yu'vath implant.
Remember the Rule of Cool and The Amendment of Awsome
The limit of the Willing Suspension of Disbelief for a given element is directly proportional to the element's awesomeness.
If something is awsome enough, it excuses everything!
Similar. It had pre-weakened the wall as it was meant to be an ambush, which would have been noticed with a successful Awareness test had anyone had that skill. It was probably a needless conceit, but it was more than a little meant to be fun. The Shaman had also ended its turn, but the other Rak'Gol hadn't and were capable of entering into melee. I had pictured the ambush as similar to the moment from Alien where the PCs had assumed the Rak'Gol were only capable of charging and dying, but were almost wiped out by reasonably straightforward ship tactics. Ship corridors are also no more than 10-15 meters wide most of the time, so it wasn't hard to stay in range.
How would Psyniscience work in that case? I had read it as you can detect a Psyker actively using or sustaining a power, but you can't just passively scan for a Psyker - that's what Mind Scan is for. As the Astropath in my campaign always sustains TK Armour and PK Shield, he made a nice glowing Psychic target and was the focal point for the ambush.