Comment We basically know nothing (Score 1) 43
This is one of the most unknowable questions there are. There is absolutely no reason to think that they have, and there is absolutely no reason to think that they haven't. I imagine an intelligent anthill trying to work out whether any other intelligences have ever come to their island by looking for unrecognised pheromone trails.
No aliens, ever, is obviously possible, it's the simplest way of explaining why none have ever said hello or left behind stuff which has no other explanation. There are huge distances involved, it might be incredibly rare for for machine-building intelligence to evolve, and there are plenty of plausible reasons for machine-builders to not end up crossing those distances.
But it also wouldn't be remotely surprising for a few or a lot of alien things to come here without us noticing. They might be so strange we don't know what we're looking for, they might have hiding technology which is just as good as their interstellar travel technology and ethical and/or legal reasons to remain hidden. Or they might have just been here a long time ago, and picked up all their litter.
Or... they may have only been around hundreds of millions of years ago, with anything left behind having long since broken down and become atoms in rocks. Humans have only been around for a tiny fraction of this planet's lifespan. There's a tendency for people to think of official representatives visiting now and doing secret deals with governments, but when someone talks about aliens having visited Earth, I imagine things like a group of scientists and thrill-seekers coming down to collect organic material samples and have fun back when mammals weren't really a thing yet.