Leaving a mobile phone in the drawer doesn’t help things as mobiles vary their transmit power based on signal strength, so if the cell site is getting a weak signal the phone can up it’s transmit power to compensate at the expense of battery charge life.
There is a free download on the Market “OS Monitor” it will run in the background as a little gauge in the status bar so you can see if a process is taking lots of CPU time (blank OK, varying levels of green for more n more CPU) If you see the gauge is high then you can open it and it will list all the processes and it will sort them based on CPU usage. This means at least you can narrow it down to a program/process that is being a bit naughty.