Thanks. I had the same thought re the currency app and deleted it as well as a weather app, thinking it might be running in the background - although tapping individual bars on the graph (which gives app activity) did not show anything untoward. Also rebooted the phone. Since then only dark blue bars appear in the graphs - which is a relief although I still would really like to know for sure what on earth was running in the background (screen off) all that time.