Is the Wireless turned on? If so, and if it couldn't find and kept trying to establish a wireless connection, the battery could drain fairly quickly. Also, how bright do you keep the display and how long a time interval do you use before the nook goes to standby when not used?
Occasionally my NookColor will lose charge quickly. I have found that when this happens the idicator that shows the battery level displays the "charging" notation and the NC is not plugged in so it can't be charging. If I don't notice this immediately the charge is used up very quickly. The first time this happened the NC was in my bag at work and I pulled it out to use it at lunch and the edge near the idicator was very warm, the charge was almost gone. The only way to interrupt this faulty "charging" indication is to shut the NC completely off and then re-start it.
I can't pinpoint any specific things that triggers this behavior since it has happened first thing after unplugging the NC and in the middle of the day after some use.