Most formatting decisions are made based on what the page looks like. That is why we usually ask to see the actual page. I can't say with certitude that you have a sidebar because I can't see it. What I have is your description, which is an interpretation of what YOU see.
Your description says that the material is in the margin and doesn't relate to anything specific in the text. Compare that to the definition of sidebar in Formats 12.2.1:
"A sidebar is detached from the main text and found in a section either off to one side of it, or above or below it, on the same page. Sidebars may or may not be linked, or related to, the adjacent text, and the reading order of sidebars and text is not always obvious." [Italics mine]
Based on your decription I would GUESS that you have a sidebar. If you agree, format it as a sidebar, according to the text layout as described in Formats. Other kinds of notes (that have a specific reference to the accompnaying text) are now called just "notes" and are used with the appropriate reference indicator and formatting as explained in Section 16. The marginal note indicator that you refer to, for notes that are now called sidebars, is indeed defunct. The trick here is to decide whether you have a note requiring a reference indicator or a note that is in fact a sidebar, which gets no reference indicator.
Does that help?