An idea I've batted about for a few years--but do not subscribe to, for the obvious reason that it's not falsifiable by its very nature--is that if they were here, we couldn't detect them. Not because they're invisible, in the conventional sense of the term, but that once they reached here (and everywhere else) they could ensure, through technological means, that whatever life evolves after them simply can't see, feel, etc. the evidence of their existence.

We don't see the real world directly. Our senses are fallible, and are filtered in our brain.
If they made it here back when our (and everything else's) ancestors were bacteria (or whatever passes for their version of primitive), they could have left behind self-reproducing micro and nanoscale technology for the purpose of being incorporated into any life that becomes too intelligent. Into our nervous systems. Could toy with our brains, and simply edit our senses in real time, our memories, and even what we're capable of thinking. No matter how far we advance, if even into hyper intelligent artificial machines and computers the size of solar systems, we would be incapable of detecting if such a thing were being done to us, since we'd have to detect it through our senses.

The reason for why they'd do such a thing is simple: absolute control. Nothing that evolves after them could destroy or otherwise make war with them.

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If we don't play god, who will?-James Watson
I never think of the future, it comes soon enough.-Albert Einstein
The large print giveth and the small print taketh away.-Tom Waits
Any sufficiently advanced technology is indistinguishable from a yo-yo.-Enoch Root, The Confusion
When I was a kid, if someone brandished a shrink gun he'd get a little bit of respect!-Myron Reducto, Harvey Birdman