1. Yes that's happened to me. Almost the exact same thing, with people disappearing. But I start panicking more over losing my bearings, though, and not being sure of where I am, rather than not being sure if the other person was there. Almost like somehow the world has shifted, I've been spun around somehow, and things don't look familiar.

2. Yes, yes! This happened to me recently, when I went off my meds. I thought there was a zombie outside my window, on my fire escape, while I was smoking a cigarette out the window. I couldn't see it, but I could feel it, and felt like I could reach out and touch it. When I went into my living room, I sensed/imagined a zombie dog outside my living room window. There is no terrace or fire escape out there, it just drops straight to the ground, but I believed it was there. I knew logically, that there were no zombies. But it was like dangling on a precipice. I believed I could reach out and touch.

It is like a hallucination, but you can't see it, but it's there. I know exactly what you're talking about.

I don't know what causes it, however.