SPAAAAACE!

Something more exotic- carbon monoxide would trigger the servitor's sensors and show up on the diagnostics, as well as trigger ship-board atmospheric alarms. Also, judging by the fact it killed cultists and servitors but not the silverfish or other space-vermin, it was somehow targeted to human genes, and set to decay after a period of time to prevent identification. So I guess rather than targeting haemoglobin, it attacked the lungs to the same effect.

Relics are gathered! There's an aquila on a chain about the priest's neck, the skulls of his predecessors, and a small reliquary containing a spent brass bolter casing, allegedly fired by a Space Marine and ornately decorated with engravings of various symbols, including a pair of crossed gladii under a jawless skull. There's a collection of religious books, but they are falling apart. The silverfish have, in fact, been eating them. The soldiers do have bags and webbing to put the relics in, though none look particularly happy at carrying a skull, even the odd ones- the one with an archaic augmetic installed in the left eyesocket, or the one that has been dipped in silver, or the one covered in religious scrimshaw. They're all quite delicate, even the youngest skull being decades old.

"Are you ready? We shouldn't dawdle. They don't know where we're going, but the longer we wait the more likely they are to get in our way." Even if the genestealers can't figure out why they're going where they're going, they could still work out the direction and get in front of them.