Recently I wanted to open my MacBook computer to check my e-mails. But I immediately noticed something strange on the lock screen (the screen where you login into the computer). There was a message being shown at the top of the screen – the so-called Menu Bar – that I had not seen before. I was wondering what is the meaning of this message?
Your screen is being observed
I wasn’t immediately alarmed, but I thought it was kind of odd that my screen was somehow being observed by another person or some sort of system. The macOS operating system is known for its privacy-oriented design, and so it wanted to warn me that whatever I do on the screen may be seen by a third party.

How to fix this message?
There could be a number of reasons why my screen was being observed.
Disable Screen Mirroring
The macOS operating system has some neat features that work perfectly between other Apple devices. One of such features is the Screen Mirroring. This functionality is installed by default as part of the operating system of your MacBook and it allows to share the screen (either mirror it, or extend the desktop) with compatible devices such as the Apple TV.
In my case, I had been sharing my screen via the Screen Mirroring on my Apple TV the day before. For some reason, that connection was still active and that was the reason why the message was displayed on top of the screen in my case. It warned me that my actions could be followed by another person who was at that moment looking at the Apple TV screen.

Screen recording on your Mac
The second possible explanation of the alert text is: screen recording.
The built-in app Screenshot.app is available on all Mac computers and laptops and is most known to take screenshots of (parts of) your screen. A bit less known is its ability to also record a video of your screen. You may have, by accident or on purpose, started a screen recording session before you locked the screen. You may open the Screenshot app by pressing CMD+Shift+5 and in case there is an active recording ongoing, you will see it there.
Again here, the message is actually a privacy feature to inform you there is a screen recording ongoing, so you should not be worried if you started the screen recording session.
Note that there are many third-party apps that have the ability to record the screen, including some games that can record your gameplay. So in case you are investigating what is the source of the ‘Your screen is being observed’ banner, you should take into considering that third-party apps (non-Apple apps) that you installed yourself are actively recording your screen.
Any third-party app will be listed in the Screen Recording section of your Security & Privacy page (open it via Settings). You can there remove the permissions from that app to start recording your screen in the future.

Inspect your computer for malware
Another option to check is to confirm no malware of any sort is installed on your computer. While this is rather rare, it could be the case and it doesn’t hurt to perform a security scan of your (system) files and processes.
The aim of the article is not to recommend the best security apps for Mac, but some known are Malwarebytes or Avast, which both offer free options and will allow you to detect any possible spyware or malware of your computer.