Within the novel The Hydrogen Sonata it is stated that several things tend to happen once a civilisation has made the decision to Sublime. Strange objects called Presences appear, Scavenger species (less technologically advanced species wishing to claim the departing species' technology) arrive, and any other civilisation that has kept anything quiet with regard to the Subliming civilisation customarily reveals it to them before they depart – a setting straight of the record for advanced civilisations.
One such species are the Zihdren and they have quite a large secret. They planted the Book of Truth, which the Gzilt revere as it predicted much of their technological developments with an accuracy not seen in any other civilisations equivalents (due to the Zihdren writing it from the perspective of having developed all those technologies), and it is implied that the religion made the Gzilt refuse to join the Culture in the past due to their feeling of being special and different from all others. However, the Zihdren themselves have long since Sublimed as a species, meaning that the Zihdren-Remnant – apparently a mix of AIs and biological Zihdren that chose not to Sublime – are left to deliver the message. However, a Gzilt warship intercepts and destroys the Zihdren-Remnant envoy ship several weeks before the Sublimation is due to take place, in order to preserve the secret.
Sources: The Hydrogen Sonata