I've seen one, and heard a couple more.
The one collaborated by others is a ghost/ mimic at my in- laws' house. To me and most everyone there, it has presented as someone else in the family. I've seen it as my niece and my mother in law, both in a place they couldn't be, and then disappearing once I realize that. MY husband is the only one who has seen the real face of the ghost, a 12 year old girl with brown hair in a sundress. In all cases, it is just translucent enough to jar the brain into thinking something's wrong.
The other was at Six Flags over Texas, in the backwards house (no longer there.) This older man just straight up walked through our tour guide like he wasn't there.
I've heard several more, usually the noises humans use when they're trying to catch someone's attention. ("Hello?," "OVER HERE!" "Aqui!")