Hey, can I come in?

Okay. So I swear this is true, even though my parents think it was a a lapse in my sanity or something. I have OCD that makes me doubt my memories a lot and let me tell you- I have never once doubted this. It's not scary as much as it's like... Unsettling? So I apologize if you guys don't like this. It's my first time posting after all!
So. This may be one of my earliest memories. I'm 7 years old and I'm in my backyard playing on my cookie-cutter swing set that you can probably get for about 50 dollars at any Home Depot. It was green and white, the green being plastic. It had a bench swing, two regular swings, and I loved it. (Unnecessary detail, I know.) And as I liked to play pretend quite a bit I was pretending I was a pirate when I saw someone walking down my street. (Note: My yard is very big. Not like sizable land or anything, but we have a big fenced in yard with about four or five trees in it. Think of the size of a College Courtyard. Sort of like that. I live in a neighborhood so it's very out of place looking. There's a street on either side of it, and a house behind it.) It was a girl about 18 or 19, hair to her jawline. It wasn't nice bobbed hair but like "I was scene a year ago and even though I'm not now I just haven't got around to a normal haircut." It was fried on the ends, ect, straightened to death. She had on the big puffy vans everyone wore back in the day, and jean shorts. I'm telling you this so you know how vivid this memory is for me. I walk up to the fence and stare out at her. (I was a weird-ass kid.)
"Hey," She said to me, stopping and looking at her cellphone. She put it away and smiled. "Can I come in?" She asked me as she looked around.
I know what you're thinking. No OP! Don't do it! Well, I did. I let her in. Luckily for me I didn't get stabbed, or molested, or like something else. We just played. And if my memory serves me right, it was for like two hours. We swung on my set, she asked me age, ect. It was super creepy looking back. Suddenly she got a text on her phone and she glanced at me with a very soft smile. "I'm going to go now. But I'll be back to play soon, okay?" I of course was like hell yes, come back new best friend. She gave me a hug, exited my fence, and walked down the road.
About five minutes later my parents come out and ask me "Who was that?" I told them who she was. My parents just being glad that I wasn't like abducted, kind of brushed it off. Then, about twenty minutes later a car pulled down the street. They explained to us they had been searching for their daughter who had gone grocery shopping but didn't come back home. It was about dusk now. I of course being a little 7 year old I didn't make the connection and they drove off.
But I did realize soon after that I have met that girl, the one who played with me. "But OP! Why is this unsettling?"
Well, it's about the layout of my street. At the end of my house's driveway the two streets on the either side of my yard merge. They go down a street that leads to a dead end. The dead end has rocks where you could park, and then if you walk down you'll reach a creek. During the summer where I live it rains a lot and so it's very dangerous, kids in the neighborhood were mostly not allowed to go near it without an adult. It's probably 6 feet deep in some parts with sharp rocks and some rapid patches.
When the girl left my house and went down the street, she was walking in the direction of the creek. If she walked back I surely would of noticed, and we have a curfew of 10 in our neighborhood so if the cops saw any teenager milling around that late they'd take them to the station. There's no way out of the neighborhood unless you come back up one of the streets. I did not see that girl come back from the creek. I did however, see her parents again. The next day they were going door to door asking if anyone had seen their daughter. She hadn't turned up.
Horror churned in my stomach, what if she had...? I was too afraid to say anything and essentially curled up under my bed with gastrointestinal distress for a day.
I don't think they ever found her. Had I been the last person to talk to her? To see her? I wasn't allowed to watch the news as a kid, and I can't find any stories on this. Which, yeah, call bullshit, but maybe they just found her, no need for a news story.
Or at least, that's what I tell myself when I look out at the rushing water adjacent from my home.