Chrome is one of the browsers that Software Updater monitors. For whatever reason when a program is updated automatically or manually before SU has been refreshed with the new version of the update, the program disappears from SU. When Avast! updates SU, Chrome will reappear.
It happened to me with FF. SU said the update was FF 19.0.1 and I updated to 19.0.2. when SU failed to show the upgrade, I rescanned it and FF disappeared. It did not show up again until next day or after when it says that the FF version was 19.0.2.