Message “Your screen is being observed.” & Apple TV

This message started today after connecting my Apple TV to my TV to use the TV as a second monitor with my MacBook Air 2013.

I confirmed this was the cause by

clicking on the Apple icon in the upper left corner of my MacBook Air,

selecting SYSTEM PREFERENCES,

selecting DISPLAY tab,

changing AIRPLAY DISPLAY from APPLE TV to OFF.

I locked my laptop screen again and the message was no longer appearing.

I repeated the steps but enabled APPLE TV again. As expected, the message appeared again after I locked the screen.

Hope this helps someone who might be experiencing the same problem.