The documentation implies that the mentioned signals are emitted when a change is done to the underlying datastore, e.g. when landmarks are removed by the Landmarks application on a Symbian device.
The LandmarkManagerEngineSymbianPrivate::handleDatabaseEvent() function found in plugins/landmarks/symbian_landmarks/src/qlandmarkmanagerengine_symbian_p.cpp simply looks unfinished as those signals could be emitted here by actually handling EPosLmEventLandmark* events.

BTW, it would be nice to have all thouse qDebug()'s removed in the release version of the software not only found in this file.