If they click on the URL while their internet is out, then the URL may remain in their browser's search history. The search history may sync between the devices

If they click on the URL while their internet is out, then the URL may remain in their browser's search history. The search history may sync between the devices, so if someone has access to your phone, they may get the secret URL when your internet resumes (and you haven't clicked yet). This probably isn't terrible though.