I want it the poor Space Marine to take forever to figure out that there’s been a mistake, and dutifully keep memorising archive systems and pushing around trolleys in the assumption that it’s all some kind of lengthy initiation process.

Meanwhile the Actual Librarians figured it out in 0.2 nanoseconds, and are gleefully doing everything they can to maintain the charade (he is SO helpful to have around, and the number of data clerks failing to return their slates on time or trying to sneak jars of caffeine into the stacks has gone WAY down since Brother Amaltheus started doing the shelving)