Mysterious Painted Mark

Outside the front door of my block there are some small metal covers, all marked 'stopcock.' Recently one of these metal covers has been mysteriously painted blue. I'm very suspicious about this. Why has it happened?

I walked down the block, looking at the other entrances. There are similar small metal covers on the pavement outside other doors. None of them have been painted blue. Or painted at all.

It all seems very suspicious to me. And a little worrying. What can it mean? At present, I'm...