Unreconciled New Layers is a notification that lets you know that new layers have been found. If you right click on the Unreconciled New Layers icon you can also access the New Layer Notification Settings.
Unreconciled New Layers is a temporary filter that shows the layers that can be reconciled using right click. Being a filter makes it also easy to review the layers before acting on them.
I understand that lately this warning has become a nuisance. Because of that, a new routine has been added to the startup of AutoCAD. The routine sets variables within each drawing that suppresses the balloon notification. Unfortunately, the setting is not global but drawing specific. When you open a drawing for the first time the balloon will appear. Close the balloon and continue with your work. When you’re done, save and close the drawing as you normally would. From this point forward anytime the drawing is opened you will not receive the balloon notification.