Detecting shower usage

My daughter tends to start dreaming under the shower and forgetting time. I'd like to do some gentle reminder after 15 minutes by blinking the lights. For this I would need a way how I can detect when somebody starts showering. I don't want a motion sensor because even though it doesn't really see something, she still would feel uncomfortable with it. Perfect would be a water consumption sensor which I could screw in between water outlet and hose but I can't find something. Any ideas? I'm in Europe / Germany and use a mixture of Wifi and Homematic based components.