There is a bug in watch OS2 which doesn't turn off the sensor properly after using Siri to invoke the heart rate monitor (according to the latest patch notes from Apple). That's probably what some of us are seeing here, I too have now noticed bad battery life (on one day) and I think I had manually used the heart monitor glance. It maybe that just the act of putting the watch back on charge kills the heart rate monitoring too, and this clears the bug, as the next time I used the watch (after charging it) it went back to getting 3 days use again.