When View -> Non-printing Characters is enabled, the Non-printing Characters are not shown in text boxes.
Confirmed in v5.2.1.2 and v4.4.0.3 / Windows 7 (didn't find any option to insert a textbox prior to that).
there is a request somewhere for non printing characters in Impress/Draw. I guess that is using ~the same layout engine?
Seems this Writer only issue would also be handled in the general case of bug 102249 in adjusting EditEngine source. Resolving dupe to there as well. https://bugs.documentfoundation.org/show_bug.cgi?id=102249#c16 *** This bug has been marked as a duplicate of bug 102249 ***