ok - after moving the cursor back and forth I got the invisible character named in the editor.
I had to put cursor on the 1st visible letter , then back arrow to get it to show at bottom right
the description then says
Object Replacement Character
so I am none the wiser as to how/why it got to be there, but I reckon I can zap it with my previous planned regex, using
not A-Z to track it down, I probably need
not A-Z or " to cover all start of sentence possibilities
here is a wiki entry: for object replacement character
https://en.wiktionary.org/wiki/%EF%BF%BC
(computing) The object replacement character, sometimes used to represent an embedded object in a document when it is converted to plain text.