The memory clips of GrayCris's crimes that Murderbot carries, meant for Dr. Mensah.
The evidence clips are the trove Murderbot smuggles off Milu and carries into book four. Stashed in its right forearm are memory clips of data drawn directly from the terraforming facility's geo-pod diggers, documenting the strange synthetics GrayCris illegally extracted under cover of a terraforming operation. More damning still is a clip Murderbot recovered from Wilken and Gerth's gear: a carefully organized record of their work history for GrayCris, apparently assembled as blackmail insurance—and now a weapon in Murderbot's hands.
Delivering these to Dr. Mensah in person is Murderbot's original mission, though her disappearance forces it to rethink how.
Small memory clips, some implanted in Murderbot's right forearm, others carried in its bag.
The MacGuffin: physical proof of GrayCris's crimes that drives Murderbot's mission.