Turning off WiFi definitely has a positive effect on battery life!
It has happened to me a couple of times that devices suddenly have gone crazy and completely drained the battery in no time at all with for obvious reason. My guess (and that's all it is!) is I may have inadvertently left some process running on it. Turned it off, recharged and restarted and voila, all is well again...
So if this battery drain was a one-off you may not need to get to nervous. If it has been for several charges, then probably more reason for concern.
Did you buy your Nook in a physical store? Bring your Nook and you just may be lucky! Sometimes they help you even when they don't have to!