Android noob here. What's the best way to find a rogue app?
Check your battery stats (settings>battery). This lists what parts of your phone are using most battery power, although it can miss things sometimes.
Android System Info is a good general indicator too, because it will show you how much time your CPU has spent at certain speeds. If it's a lot of time at full speed and you've barely used it, then something is acting up.
If you want to narrow it down a bit, download a CPU usage monitor from the Market and see what apps are using most power.