I have a Tech Priest in my current campaign and I'm not entirely sure what to do with him. I would have preferred not to allow a PC Tech Priest at all but it was the career that mostly strongly interested the player.
My problem is this: I've always understood that the Adeptus Mechanicus are essentially an empire within an empire as far as the Imperium is concerned. In many ways they are an independent organization, distinct from but allied with the Adeptus Terra. As such, I see the Tech Priest as a free agent... almost a foreign citizen... who can't really be ordered around in the same way as the other characters. In fact, I think the "foreigner" aspect was part of what appealed to the player. He has a habit of playing the sardonic outsider in any rpg I've had him in (and in real life come to that).
Perhaps to complicate matters further, the characters in my game aren't acolytes of the Inquisition (so I can't fall back on 'the Inquisitor said so" even if I felt it was appropriate). The other characters are members of the the local Planetary Defense Force, currently assigned as members of a nobleman's command cadre. The Tech Priest was originally assigned to the same nobleman's household as an enginseer overseeing the house's manufactories and mechanical infrastructure, and became involved with the PDF troopers while jointly investigating an assassination-by-sabotage.
The investigation is (mostly) resolved now. Unfortunately, I'm not really sure what holds the group together now. I'm trying to figure out an IC reason to continue to include the Tech Priest in the group. Thoughts and suggestions would be greatly appreciated.