What iOS version did you have prior to 9.2 beta? Could've been that beta 2 was still settling in for a while. Now that it's settled, that's probably why you're seeing the improved battery performance.
I was on 9.2 beta 1 prior. Like you say, I'm sure beta 2 must have triggered something around background indexing for a day or two, although according to battery log it was the phone app using the highest % even though my phone usage was actually low on those days.