I just realized - now that Tuuri's (possibly) infected and has to be kept separated from Reynir, Tuuri/Sigrun alone time is much easier to organize. Since Reynir is stuck in the part of the tank where everyone usually sleeps, Tuuri is clearly sleeping somewhere else. Probably the driver's seat, since that looks reasonably comfy? And clearly she can't be left alllll alone, so Sigrun keeping her company just makes sense!