I was cycling through Hyde Park on my way to work last week on a cold wet morning and I had to break to allow a jogger to cross the path. It was none other than....Madonna! She gave me a nice smile and a cheery "good morning". Considering what most people say about her here it was a pleasent surprise.
Maybe she was happy you didn't instantly whip out an autograph book or a camera? It's probably really rare for her to encounter "normal" people who don't try and shout at her for a signature or photo.