Starting with NVIDIA GeForce Experience update 3.9, the NVIDIA icon has been removed from the Windows System Tray (Clock area) "To reduce the system footprint of NVIDIA software" as stated in the GeForce Experience 3.9 Release Highlights.
For those that want the NVIDIA icon added to the Windows System Tray again, for whatever reason, may do so by following the procedure below.
1. Download and unzip/extract the NvTrayIconRestore package
here from my OneDrive page..Readme instructions are included
2. Create a Windows System Restore point (backs up registry)
3. From the NvTrayIconRestore package, copy and paste the ShowNvTray.reg and the NvGFTrayPlugin.dll and the NvGFTrayPluginr.dll files into the C:\Program Files\NVIDIA Corporation\Update Core folder..Substitute your Drive Letter if other than C:\
4. Double-click to run the ShowNvTray.reg file which will auto-add new Windows registry entries if you agree
5. Open the NVIDIA Control Panel, then the Desktop dropdown (top-left), then check to Show Notification Tray Icon
The Notification Tray Icon should now appear in the Windows System Tray and after Windows start up where you can left-click to open the NVIDIA Control Panel or right-click (for advanced options) and choose to Open NVIDIA GeForce Experience or Open NVIDIA Control Panel or Check For (driver) Updates as well as set (driver) Update Notifications.
The NvGFTrayPlugin.dll and NvGFTrayPluginr.dll files are from previous versions of GeForce Experience that showed the NvTray icon in the Windows System Tray but were removed by NVIDIA..The ShowNvTray.reg file auto-adds the correct registry entries for you to show the NVIDIA Tray Icon again in the Windows System Tray.
Updating/removing/reinstalling NVIDIA GeForce Experience will remove the Windows System Tray icon for GeForce Experience (but not for NVIDIA Control Panel) and this NvTrayIconRestore procedure will need to be repeated to show it in it's entirety again.
Should you wish to remove the NVIDIA icon added to the Windows System Tray, either go back to a restore point, reinstall or update NVIDIA GeForce Experience, or edit the Windows Registry by pressing the Windows + R keys, then type regedit in the box, then click OK, then navigate to Computer\HKEY_LOCAL_MACHINE\SOFTWARE\NVIDIA Corporation and delete the NvTray entry.
NVIDIA has apparently done away with the NvTray icon for a reason, but I have noticed no adverse effects whether is shows or not..Your experience may be different.