The Problem Child was an early General Contact Unit, Troubadour Class, which chanced upon the ember of a very old star, near the Asatiel cluster, indicating it was about a trillion years old. The ship was so embarrassed about what it was coming up with it never filed a full report, just kept the results to itself, in its own mind.
It also discovered a perfect black-body sphere fifty klicks across, in orbit around the ancient star. The ship was completely unable to penetrate the artifact with its sensors, and the thing itself showed no signs of life. After developing an engine fault (later discovered as the result of offensive action), the ship had to leave the star, leaving a load of satellites and sensor platforms behind it to monitor the artifact; when a follow up expedition arrived three years later, it found nothing, no star, no artifact, and none of the sensors and satellites the Problem Child had left behind.
Over the following seven decades, the Problem Child decided it didn't want to be part of Contact any more. It left Contact, then it left the Culture proper and joined the Ulterior. The ship’s drones all joined the same Group Mind in the Ulterior and have been incommunicado ever since. The Ulterior, and Special Circumstances lost touch with the Problem Child entirely. That was fifteen hundred years ago.
Every single human who’d been on board at the time exercised Unusual Life Choices. Roughly half of the humans opted for immortality, the other half autoeuthanised. Within a century, almost all of those humans who’d opted for immortality were also dead, due to further “semi-contradictory” Unusual Life Choices.
The few remaining humans underwent subtle but exhaustive investigation, though nothing unusual was ever discovered. Subsequent investigations of the remains of a few of the humans concerned, using improved technology, has thrown up possible discrepancies in the nanostructure of the subjects’ brains, but no further investigation has been deemed possible.