Found: Several "operating system command" characters!
Save this for your next bout of insomnia ...
Made an interesting discovery upgrading from Arbortext m010 to m090. We found occurrences of the following Unicode character U+009D in our files. I am setting up a hunt and destroy mission so I do not have exhaustive information but it appears that they show up in our mainframe screen captures. I suspect the characters have been there all along but that previous versions of Editor and Publishing Engine have gracefully (to me, perhaps buggily to Unicodologists) ignored them.
We discovered them through FOSI formatting errors reporting characters that were unavailable in Courier New and that they were being replaced with Arial MS Unicode. (No matter, formatting-wise, they have no output impact.) PE complains about something-or-other(157) and yes, the way to create this character, in Editor, is to type alt-0157 using the number pad. Or, in my case fn-alt-0157 using the in-laptop-keyboard-fn-number pad. If you know what I mean.
By creating an instance of this character, I could copy it to the Find/Replace dialog and replace all occurrences with null. Problem solved.
When you encounter this character in text, it is invisible. You can watch your cursor do weird things if you navigate across one or more of them (and they did seem to appear in small multiples) but otherwise, you can't see them there, in Editor, any more than you could see them in the PDF.
Hmmm. Just realized, I never tried looking for them in a PDF. Probably won't. Don't care.
While typing this up, I realized it is possible that the fonts installed on our m010 and m090 PE servers were not necessarily identical. (I don't know for sure. I don't care. Won't be checking.)