Power Drain

[CyberMonkey]

Good day all.
Need to ask a question, I am in a training camp this week, and today was the first day.
Phone had normal use, had some missed calls and txt messages that I could not answer/reply to.
In the afternoon, power level was at 70-ish/80-ish. When they ended the session, power level was at 30%, there was no usage from last I checked and it was probably 2 hours or so that it had gone from 70-ish/80-ish down to 30.
Any ideas of why that power drain could have occured?
Where I was was, the phone has strong signal. Oh and speaking of strong signal, it's great being in a city that the fast web data is actualy useable.