This power drain sometimes happens to me, when it does i know about it because the phone will be hot in the morning; I usually keep it on charge overnight and under the pillow. Even if it is not on charge and the this happens the phone will heat up, it can happen even with no apps open.

Checking the "Idle Consumption" I've seen as much as 320mA being used.

A reboot has always fix it.

I have also noticed that even with no problems the N9 gets warm when under the pillow, something the N900 never does.