Missing the Airplay/Display icon in the top menu bar
I have an iMac (21.5-inch, Late 2011) running El Capitan, that is missing the Airplay/Display icon in the top menu bar. This affects all user accounts, admin, standard and guest. The check box to Show mirroring options in the Sys Pref is checked.
I've tried rebooting and also resetting the PRAM. I've also gone in to HD->System->Library->CoreServices->MeunExtras and opening Displays.menu. This does bring the icon back and I can click on it to open Display Preferences, but as soon as the Sys Pref opens, the icon is gone again.
It is plugged into a projector via Display Port to vga adapter -> vga cable ->wall plug. I did switch out the wall plug yesterday because of a faulty audio port, but I can't see how a different wall plug would make the icon disappear.
I guess I'll try a OS reinstall in place next, but any other ideas? TIA
iMac, OS X El Capitan (10.11.6)