There may be many elements (classes, components, etc.) in many diagrams. Some of the elements may have notes, others not. As I do not see which elements are furnished with notes, I have to hover over them, or I have to click on them and look at the notes tab.
This is kind of a trial and error search. Consequently, I might miss to read an important note.
The trial method is a bit cumbersome if models are passed around a team. Team members have to look out for notes, but do not know if it is worthwhile to examine an element closer. Just think about the situation when a developer passes a model to product manager: Information might be overlooked.
Another aspect is the generation of reports in HTML format. There, it is especially tedious to find all notes.
So, I would suggest to give a graphical hint for the existence of notes. In addition, as mentioned in a thread before, I would like to have tool tip texts showing notes when hovering over an element.
The graphical hint might be an icon, or other graphical elements as color. The indication might be permanent (useful for reports), or it might be enabled by the user.