I noticed this when using a sideways case that had a magnet in it. My battery life totally sucked.

The magnet is making the phone think the keyboard is open. If you turn on the onscreen keyboard and close the slider - place a magnet at the back of the device then do a dmesg from the shell you will see the message:

slide (GPIO 71) is now open

So the phone thinks the keyboard is open when it's not.

Sucks for me since I liked the case with the magnet.