The note not found contains normal text. As mentioned, every word in the note cannot be found. I understand that you cannot reproduce it, but I read in the forum that several people are experiencing the same problem.
I discovered the problem with one note, but there may be more notes that cannot be searched for. That's the tricky part. You don't know if you're missing something if it isn't found. The incident mentioned is the only one I know of.
You say: "it seems unlikely to be a general note issue". I don't exactly understand what you mean by that.