I have worked it out.
See non printing characters.odt.
I opened Notepad++ and went Edit > Character panel. This opened a panel with 255 characters a number of which did not display. I copied a few non displaying ones and pasted them into a document and saved it as non printing characters.odt. As you can see, only the " # " displays, which I think was ACK, but content.xml shows the other hidden characters (DCS, DCS, SS3, RI and HOP) are in the XML.
You can easily add them to any text by setting up an AutoCorrect where, say, %%M1 is replaced by, say, DCS, which you paste into the Replace box. Typing %%M1 now gets your
Marker_1 inserted as DCS. Or use AutoHotKey to do the same thing.
EDIT: I could see the markers in the code below when I wrote the post (and if I edit the post) ... but as they are non-printing, they don't display here either

You will have to extract content.xml to see them, or copy from ">" to "#" to copy them.
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<?xml version="1.0" encoding="UTF-8"?>
<office:document-content xmlns:office="urn:oasis:names:tc:opendocument:xmlns:office:1.0" xmlns:style="urn:oasis:names:tc:opendocument:xmlns:style:1.0" xmlns:text="urn:oasis:names:tc:opendocument:xmlns:text:1.0" xmlns:table="urn:oasis:names:tc:opendocument:xmlns:table:1.0" xmlns:draw="urn:oasis:names:tc:opendocument:xmlns:drawing:1.0" xmlns:fo="urn:oasis:names:tc:opendocument:xmlns:xsl-fo-compatible:1.0" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:meta="urn:oasis:names:tc:opendocument:xmlns:meta:1.0" xmlns:number="urn:oasis:names:tc:opendocument:xmlns:datastyle:1.0" xmlns:svg="urn:oasis:names:tc:opendocument:xmlns:svg-compatible:1.0" xmlns:chart="urn:oasis:names:tc:opendocument:xmlns:chart:1.0" xmlns:dr3d="urn:oasis:names:tc:opendocument:xmlns:dr3d:1.0" xmlns:math="http://www.w3.org/1998/Math/MathML" xmlns:form="urn:oasis:names:tc:opendocument:xmlns:form:1.0" xmlns:script="urn:oasis:names:tc:opendocument:xmlns:script:1.0" xmlns:ooo="http://openoffice.org/2004/office" xmlns:ooow="http://openoffice.org/2004/writer" xmlns:oooc="http://openoffice.org/2004/calc" xmlns:dom="http://www.w3.org/2001/xml-events" xmlns:xforms="http://www.w3.org/2002/xforms" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:rpt="http://openoffice.org/2005/report" xmlns:of="urn:oasis:names:tc:opendocument:xmlns:of:1.2" xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:grddl="http://www.w3.org/2003/g/data-view#" xmlns:tableooo="http://openoffice.org/2009/table" xmlns:textooo="http://openoffice.org/2013/office" xmlns:field="urn:openoffice:names:experimental:ooo-ms-interop:xmlns:field:1.0" office:version="1.2">
<office:scripts/>
<office:font-face-decls>
<style:font-face style:name="Mangal1" svg:font-family="Mangal"/>
<style:font-face style:name="Times New Roman" svg:font-family="'Times New Roman'" style:font-family-generic="roman" style:font-pitch="variable"/>
<style:font-face style:name="Arial" svg:font-family="Arial" style:font-family-generic="swiss" style:font-pitch="variable"/>
<style:font-face style:name="Mangal" svg:font-family="Mangal" style:font-family-generic="system" style:font-pitch="variable"/>
<style:font-face style:name="Microsoft YaHei" svg:font-family="'Microsoft YaHei'" style:font-family-generic="system" style:font-pitch="variable"/>
<style:font-face style:name="SimSun" svg:font-family="SimSun" style:font-family-generic="system" style:font-pitch="variable"/>
</office:font-face-decls>
<office:automatic-styles/>
<office:body>
<office:text>
<text:sequence-decls>
<text:sequence-decl text:display-outline-level="0" text:name="Illustration"/>
<text:sequence-decl text:display-outline-level="0" text:name="Table"/>
<text:sequence-decl text:display-outline-level="0" text:name="Text"/>
<text:sequence-decl text:display-outline-level="0" text:name="Drawing"/>
</text:sequence-decls>
<text:p text:style-name="Standard">#</text:p>
</office:text>
</office:body>
</office:document-content>
