Here is one story from my childhood.
I used to be in girl scouts (or "girl guides" as it is called in some places). I had a lot of fun, though I was also kind of an outcast (just like I was at school and everywhere else!). Usually I had only one friend in my troop, who was another shy "outcast", and I would partner up with them and stuff.
Anyway, we went camping a lot, and I loved these camping trips. Usually we would make tents but on a number of occasions, our troop would rent a cabin for a weekend (specifically meant for scouts, I think) with another troop, so there were new faces.
On one of these trips, we were having a camp fire with songs in our pajamas. There were about 30 of us total. One of the girls needed to go in and get her sweater from her bag. Since you always have to go EVERYWHERE with a buddy, I was asked to go with her, even though the cabin was RIGHT there. We went into the room where we all had our sleeping bags set up and as we did, an auburnish-haired girl around our age (about nine?) walked out and passed us towards the door to go out. I didn't recognize her but that wasn't too weird, since like I said there were a bunch of new faces and it was the first night. But what was odd is that she was alone and without a buddy. Also she was wearing her girl scouts sash which was strange because like I said, we were in our PJs. Some girls slept in plain clothes but none of them were wearing their uniforms by that point.
Well, we went in and got my troop mate's sweater out of her bag and then we went out again. Joined the campfire, sang the songs and the leaders told us a couple stories (probably not ghost stories though, LOL... They were usually like "scout stories". The ghost stories we kids reserved for later when we were supposed to be sleeping). It was fun and also kind of dark and I didn't really think much about that girl. I *think* I remember at some point seeing her sitting around the campfire with everyone else, but I'm still not sure about that and it was dark (we were only allowed to sit so close). So I think it is possible that that part is just my mind fabricating things or filling in blanks. So whatever... Eventually we went to bed.
The next day though, I noticed that that girl was NOT among us because we dressed up to practice our ceremony and most of the girls had vests instead of sashes. But I had a sash, being like always the awkward-one-out that I was, and I looked around to see who else had a sash. Only a couple other girls did but I couldn't find the one I'd seen the night before. And apparently no one was missing because we'd done our count off that day and everything was fine. I even asked my troop mate about it, the one who had been with me while the girl passed us, and she said yeah she remembered it as well but the girl was nowhere to be seen. I asked someone in the other troop about it and when I described the girl she was like, "You mean Theresa?"... But it wasn't Theresa, who was another girl with reddish brown hair.
I told our troop about it and they all just thought I was pulling their leg or that I was "being weird" "like usual" (I never made up stories like that but I did do other weird things). Then when I asked my troop mate for support she changed her story to say "Well, I thought maybe I saw someone but I'm not really sure." Probably to fit in with the others, I don't know.
I was the kind of kid who tended to put her foot in her mouth a lot, and I was VERY INSISTENT about things, especially when I thought I was being dismissed or ignored. So I kept pressing about it until they got annoyed at me and eventually my feelings got hurt so I dropped it and sulked awhile. And I had a really hard time sleeping that night and did not have fun with the ghost stories.
I have a few other stories but this is the only one in which I possibly actually saw something (or someone) in a solid form.