an awkward teenage moment that wasn't mine

A little over a year ago we moved to our new house, which sits on several wooded acres. Imagine a 50-acre subdivision, where most of the homeowners have 1-to-2 acre plots around the outer edge of the 50 acres...well, we and our neighbors each own about half of the remaining interior portion of the property. We live on a hill, so when we go out back there's a trail that leads down onto our property. We have some walking trails and a lovely stream with a footbridge. Our property is not fenced, and it does butt up against the rear property lines of all the homes that sit on the edges.

What am I getting at here? Well, occasionally we will look out our back windows and see people walking through the woods...which means they're on our property. It's not so much that we care if someone walks through, except that sometimes they bring their dogs (and our dog is fairly territorial) and also if someone were to get hurt (by, say, falling off the footbridge into the rocky stream and splitting their head open), I'm guessing we would be liable.

So, today I was about to take Maia for a walk, and I was planning on taking the stroller out front onto the street. But then I noticed a red-cloaked figure down by the stream. So I popped the baby into my wrap carrier and went down the trail into the woods. From my window it had appeared that the person was near the stream, maybe bending down picking plants? However, I was not quite prepared for what I found when I got down there. I approached the footbridge and saw that the person was IN the stream. SITTING in the stream, facing away from me. Near the bridge was a backpack with some towels. I said "hi, can I help you?" and an awkward-looking, chubby teenage girl turned around looking a little startled. She got up and I took in what she was wearing--a red hooded sweatshirt on top, and on the bottom some very short plaid shorts and sneakers. Did I mention she had been sitting IN the water? It's warming up here but it's not THAT warm!! It was about 55 degrees here today and our stream is in the shade.

I felt bad for her immediately. I had caught her doing something rather strange and I'm sure she was embarrassed. I said "it's a little cold for that, isn't it? I just wanted to let you know that this is private property. It's just that if someone got hurt down here we could get in trouble." By this point she had already stood up and was walking toward her towels. Not wanting to humiliate her any further, I kept walking up the trail. By the time I looped back around she was on her way off the property.

I felt bad, like I should have told her it was OK to stay or something, but what I'm really dying to know is WHAT THE HELL WAS SHE DOING?!?