Hello nicolasfox,
Thank you for using Apple Support Communities. It sounds like you've received a notification that your screen is being observed. We'll do our best to asssit.
Is this a personal, work or school owned Mac? We ask because you may have management profiles installed on your Mac for a business or school. To check your settings and confirm your screen is secure, we'd suggest reviewing the following: Turn Mac screen sharing on or off
If this is a school or business issued device, we'd recommend contacting their IT department for further support with disabling screen sharing on your Mac, if possible.
Wishing you the best, take care!