Check the load of your CPU
This less than 100 kB app gives you the load of your CPU, it refreshes every 500 ms. This is an excellent tool to find out whether you have processes running in the background draining your battery and maybe spying you.
Normally, you should see the CPU load fluctuating between 0 and 35%. If it stays at high values for a long period, an update may be running in the background. If no updates are taking place, go to Settings -> Battery and check who is draining your battery.