I just realised, unless the icon glyph data (the vector file of the icon) was somehow itself embedded in the text file, which I highly doubt, doesn't this imply that the font I was using to display the file (Adobe Source Code Pro from the adobe-source-code-pro-fonts package) has the icon glyph in its TTF/OTF file?
In that case, I'd say all fonts should be considered "suspected partially proprietary" and should be considered high-priority for an audit.