One of our Server Monitoring Agent features is that it will try to determine whether or not your server requires a reboot, as explained here:

However, the icons this feature generates might be distracting at times, so we’ve implemented a way that you can tone these down, so they only appear when opening the server metrics or disable them completely if you do not wish to be reminded of the server reboots.

The setting for the ‘Reboot Required’ icons can be found in your Account Settings, under the ‘Uptime’ tab:
(look for “Server ‘Reboot Required’ warning icon”)