There we go. The source.
Any and all such messages in a web browser are lies. Period. There is no software in the world that can tell what's on your computer through a web browser. Not even Apple knows what's on your system.
By default, Safari automatically loads whatever sites you last had open. So every time you launch Safari, you see this nonsense again. Close Safari. Hold down the shift key and launch it again. The shift key tells Safari not reload any pages from the previous session.
Whether not it reappears depend on where you go on the web. Just as with any other site, such pages are all specific to where you go. They don't, and can't appear randomly.