I had this once with a Google Chrome extension. Strange advertisements started to appear on my search results in Google. These advertisements were uncanny. After looking for this behaviour it leads me to infected Chrome extensions.

It was an extension to show favourites in a menu, nothing fancy, I had this one installed for years. It was sell by the author to a company injecting advertisements.

Now, I only install UBlock origins and nothing more.

You can play Factorio in coop? Is it good? This games looks like very personnal.