Yesterday I tried to insert Z3ta+ but couldn't find it. It was missing from the list. I couldn't find it in Plugin Manager and it was greyed out in various plugin layouts. I checked Windows Explorer and it appeared to be present in the within the programs folder and the DLL file was present in the appropriate VST folder. Weird how it was not showing up in Sonar! I re-scanned but that didn't make any difference. I launched Sonar X3 and it was missing there too. X2 also.
So I re-installed it from an old separate copy that I purchased back in 2007 before it became part of the Sonar bundle. It's version 1.5.3 but I'm not sure whether that is the same version as the one included with Sonar.
Anyway it's all ok now but it's a mystery why it disappeared.