I remember the Far Side with the guy screaming on a street corner “Vampires ! They’re everywhere ! Vampires !” Some workers are passing by with a large windowpane - and he’s the only person reflected in it. 
What do you mean, are there other societies among us, unnoticed?
What about animals? How about the complex society of the
cats in your neighbourhood? As I type at my computer, I see the cat of the people next door sitting near my catflap, looking into my house. My own cat paces nervously inside, on the other side of the catflap. The glances they exchange: pure drama!
Or how about the wet cloth I use to wipe down my worktop? I replace it every week. Every week sees, on a different timescale of our own, the rising and fall of another empire of fungae and bacteria on the cloth.
Sure, but it’s not easy. I mean, what do you think Mr. Weasley spends all his days dealing with?
maybe they’re here and they sit around devising spooky message board threads - this damn thing is giving me the heebie jeebies.
waves hand
these are not the aliens you are looking for
Heh; I recall something very close to that in one of Mercedes Lackey’s elves-in-the-modern-world novels. When asked how they kept from being discovered, one said that they “do the ‘Obi Wan Kenobi thing’, you know,” < waves hand in air > “These are not the elves you’re looking for”.
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ok, now I am suspicious.
Oh, Mr. Larson, you were so ahead of your time. Human beings tend to refuse to see, or misunderstand, the evidence of aliens!
He’s on to us. We’ll have to erase his memory and start over.