My youngest has always been a talker, but she also would lean over and "mime"(?) picking things up, putting them back down, etc. I thought she had outgrown it, until about 6 months ago (she is 9) She actually came out of her room, talked to her dad in the upstairs bathroom, told him that she forgot something downstairs, walked down the hall, down the stairs, into the living room, and sat down on the couch. I asked her what she was doing, and she said, "getting my blanket" streching out her empty arms to "show" me! Needless to say, I realized she was sleepwalking, and I herded her back upstairs, and back into bed. Definitely a mind blowing experience!
I *think* it could have something to do with night lights-- she used to use one when she was smaller, and we now leave the hall way light on. We really have nopositive cause for this, but my sister in law was a sleepwalker, too... My daughter never remembers, and never seems excessively tired the next day. Good luck!