That's true. However, there wouldn't be much evidence if there's only a relatively small amount of them, or they're really good at hiding their activity. It's a

That's true. However, there wouldn't be much evidence if there's only a relatively small amount of them, or they're really good at hiding their activity. It's also possible that their civilization peaked millions of years ago and they are now mostly existing in some kind of post-biology state, which wouldn't show any signs to the surface.