Never been to the Stanley myself (bit of a trek from Cumbria!) but I've been obsessed with it for years. My mate went a few years back and said room 217 gave him a proper uneasy feeling the entire night, like someone was standing just out of sight. He's not even into this stuff normally, which made it more convincing to me.
What gets me is how many completely separate people report the same specific things - the piano, the kids laughing in the corridors. When you get that kind of consistency across strangers who haven't compared notes beforehand, thats when I start taking it seriously rather than writing it off.
What happened to you? Was it something you saw or more of a feeling? I find the atmospheric experiences almost more interesting than the visual ones because they're harder to explain away. Really curious what you picked up on.