My guess would be that your document contains an unused style definition that uses this font. ArialUnicodeMS is also often used as the default font in MS Words docs originally created with non-US Office versions.
Since .docx files are actually .zip files with a bunch of .xml files in them, you can rename a copy of your .docx file to .zip, unzip all files and then use a free text search tool, for example,
Agent Ransack, to find out where this font is referenced in your .docx file.