Alex Graveley, the Chief Architect at GitHub on Copilot, says pretty definitively here that it does not look at anything outside your project: https://twitter.com/orph/status/1457790239796199424 So I'm thinking either it made a very good guess, or the assets got included in your project without you realizing?
> We do try to assemble prompts based on contextual understanding, e.g. if you explicitly import sources by path from some other location Copilot might include it. But not for html (yet), as is being described here.