This happened to me just this morning:
I feed the dirty siphoned water water from my fish tank to my plants, so I leave it in a bucket on the porch and scoop water out when needed. My last water change was nearly a week ago and ever since then, every time I walked past the bucket I saw a little bit of the sludge that settled at the bottom shift around. At first I thought, there must be something alive in there, but all I could see was dead plant leaves and the sludge at the bottom. So I dismissed the whole thing, thinking I must have just hit the bucket with my foot without noticing, and it had disturbed the sediment. This continued over the course of the week, and every time I just assumed I must have bumped it, although some of the times I was almost certain I hadn't.
This morning, I decided it was time for another water change, so I went outside to pour the remaining water out of the bucket. As I walked up to it, the sediment moved as it had before, only this time I was positive I had not touched the bucket. I kicked it, and the sediment moved again in the same way. But after kicking it a few more times, the sediment stopped moving each time I kicked it, and I knew there was something moving in there.
My first thought was that there had been some kind of worm or strange thing living in my aquarium and I had sucked it up with my gravel vac. I was a little bit horrified and disgusted. I leaned in closer and to my surprise, I saw a little pale head with big dark eyes on top peering right back at me. I realized then that I had sucked up my smallest ghost shrimp with my gravel vac and he had been living in that bucket for nearly a week! I was shocked, but it explained the strange movement I had been seeing whenever I came near the bucket.
I immediately tried to scoop him out and when I did, I realized just how cold the water was. Poor guy! It couldn't have been much over 60. It was difficult to get him out, because I kept stirring up the sediment and he would disappear. I finally got him in a cup and he's inside now, warming up, and he seems to be perfectly fine.
So that's my ghost shrimp story. :P