It might be injected by an app. Maybe something called "CodeMirror"? (referenced from file)
Be careful with "unused CSS". Sometimes that's a false warning because Shopify is built on a templating system. So for example if there's a CSS file being loaded it might not be used on the page you're testing, but used on a different page. You can do conditional loading within your theme code to only show it on the page being loaded if that's the case.
Another case I see something like this is if you uninstall an app but don't remove the code from your theme (once you install an app, the app loses connection to your store so cannot uninstall itself fully if it had added code to your theme).
Most of the time you can find old app code by analyzing the loaded resources on your page and comparing vs apps loaded, then check theme.liquid to make sure there aren't any old app snippets, scripts, or stylesheets being loaded.
Founder of
Speed Boostr (Shopify optimization experts, theme customization, custom app development).
Creator of
Shopify Analyzer (first performance analysis tool for Shopify sites, free for the Shopify community).
Creator of
Order Automator (auto tag orders and customers + auto fulfill orders + email notifications).
Creator of
Theme Scientist (A/B testing app) and
Tip Jar (add a tip button to your store).
