Warin made very good point when he mentioned GPS. GPS is a 'receive only' function on a phone, hence it is possible that the phone might continue to look for GPS satellites even when the transmitting functions have been turned off (in other words, it has been put into 'Airplane Mode').

It's not always obvious to the phone user that the phone is using its GPS function to try and get a location fix, because smartphone applications such as the camera, social network applications, and so forth often poll the phone silently, in the background, for a GPS-sourced location fix. This means that even if you are not running a mapping application or navigation application, the GPS engine in the phone could be running, and GPS uses a heck of a lot of power.

How to go about minimizing GPS use depends very much on what kind of phone you have, and what applications you have installed on it. On an iPhone, it is possible to go into the "Settings" menu and turn off location based services on an application by application basis. I am going to guess that other smartphone operating systems probably offer similar levels of control.

If you don't use the phone for mapping or navigation, you might want to consider turning off GPS services globally (if the phone permits that). Applications that need to know your position to function correctly (for example, TripAdvisor searching for 'hotels near me') can usually get a pretty good approximation of position - within about 100 meters or so - by triangulating cell phone towers that the phone can connect to. This gives a more than satisfactory position fix for just about any location based application other than mapping or navigation.

Michael