It most likely is due to some background apps running that want to contact a host for updates.
Social networks are bad about that. Facebook Twitter, etc... turn them off
Amazon Suites is horrible, turn OFF anything that has Amazon in the name.
email apps that have notifications turned on and are constantly updating are also bad.
I personally turn OFF all outside world contact when not using my phone.
Data, WiFi, sync, are OFF.
I also turn off Bluetooth when not using that. Why let it burn up battery if you don't use it?
GPS... I have discovered that just leaving it ON in High Accuracy Mode is not a bad thing. The total effect is about 20-25 minutes difference in battery life between ON or OFF.
Rotation? your choice, but I turn it off.
for me, a control freak about this, I use Battery Doctor's Switch Widget.
See screenshot. In this screen shot, everything is shown ON because the green color shows up better than the OFF color which is a faint grey.