The bathroom in question was long and narrow and had two pictures of some stylized city, rendered in ceramic. The buildings were unglazed clay and the rooftops and domes were in bright orange. The bathroom always smelled like my grandmother's body powder, which I learned much later smelled like freesias.
It seemed to me, looking in the enormous bathroom mirror, that I could see every part of the bathroom except the spot directly behind me, so that was where the unseen creature must be standing.
I didn't know what it looked like. I had a vague feeling it was grey and shadowy and very flat, with long arms. I thought it would probably have eyes, but no mouth, but that was only a guess.
If I moved suddenly, it moved with me. At first, I thought it was just much faster than me, but that seemed sort of improbable--and when my mother would come into the bathroom, it wouldn't matter how fast it was, it might risk being caught because there wouldn't be any place it could stand that one of us couldn't see it.
I decided that it must be able to see the future. I wasn't sure if it could see my entire future, but it could certainly see a good few minutes in advance. If somebody else came into the bathroom, it would know long before it happened, so it would flow into the hidden spot behind the door, and it would know when the people were both looking in the same direction, and it would slip out the door and wait somewhere safe, like the top of the cupboards, or the hall closet. You could probably fit a number of them in the hall closet.
The bathroom was the place it was closest to me, because the only spot I couldn't see was so close behind me. It was also the only place I gave it much thought, because I had the attention span of a small child and unless I was actively staring in the mirror, I was easily distracted by books or Legos or my grandmother's collection of brass animals.
I was a little unsettled by the presence of the unseen thing, but I wasn't actually afraid. I was actually rather proud of myself for having figured out how it worked. Then it occurred to me that maybe they didn't so much tell the future as read our minds so that it knew what we would do next, and then that it must know that I knew it existed. I wondered if my thing and other people's things would get together and discuss that a human had worked out how they were hiding just out of sight. (Everyone must have a thing of their own, of course. My mother's would be larger than mine, and my grandmother's slower and probably inclined to hang out in the family room and not get up much.)
The notion of my own predestination did not particularly bother me. God already knew what I was going to do next, so presumably other things could too. I was a little more concerned that they might have to pull me aside, since now I knew, and tell me that I couldn't tell anyone. I assumed this meeting would also happen in the hall closet.
This thing was much more benign than whatever shadowy things lurk in the mirror when it's dark (which I am still somewhat afraid of to this day.) They just hid near you. Guardian angels were part of my vague cosmology, and adults were positively soppy about guardian angels, so obviously it was fine if there was something assigned to you that followed you around all the time. It didn't seem to be hurting anything, and I was frequently alone with it, so if it was going to do something bad, it would have already done it. And my mother had one, and she was a grown-up, so they must follow you around for years and years without doing anything.
Eventually I grew up and forgot all about it.
(My publisher got me a copy of "Deep Dark Fears" for Christmas and one of them is about a thing standing just behind you, and that clicked off a whole chain of memories. I particularly remember that I had worked out that the thing must be able to see the future. I could not have articulated this to any adult very well, but it was very clear in my head.)