All server installations, included NoMachine (free), have the automatic check for updates enabled.
This means that the server will periodically check our repositories (by default every two days) to verify if updates are available. In this case, NoMachine will prompt a dialog informing the user that a new version is available but it will never automatically update the current installation.
Checking for updates can be disabled in that dialog by selecting the "Don't ask again for this version" option.
A Guide for handling the automatic updates with the proper NoMachine version is available here:
NoMachine v. 6
NoMachine v. 5
NoMachine 4 (since v. 4.6.3):