Personally, I don't know. It could be that your child is extremely observant and unconsciously gathering clues from all over the place (the NPR story playing in the car, a comment by a neighbor, something you said on the phone after the child was in bed, etc... little things that you probably don't even remember or realize she is exposed to). It could be an active imagination. It could be she is tuning into energies or entities that you're not picking up on (so "psychic" or perhaps related to a prior existance of her own).
In any case I'd probably respond in the same way... supportive and interested, open to a conversation if that's what the child wanted, but I wouldn't dwell on it or bring it up independently of the child or make much of it in front of them. If they get scared, then that's a different story of course! But if it's just a part of their life then... it's just a part of their life.
My dd2 is very sensitive to energies and entities and easily upset. I have found (as a pagan mama) that it's in her best interest for me to apply the same rules to the unseen world as I do to the seen world.
Basically, energies and entities that are interested in playing with my child need to be polite and ask permission, whether that is a physical child asking to play in the yard or an energy asking to play in the living room ceiling. I'm not cool with beings (physical or otherwise) wandering through the house without asking permission first!
So I do actively ward the house, and my dd2, and since she is 3yo I don't ask her permission to do so (any more than I ask her permission before asking the repair guy for ID before he comes in). As she gets older I'll involve her more in the process and I assume that she'll gradually take over this aspect of her social life the same way she'll take over the other portions. I had a thread about a year ago? Maybe two years ago here in Spirituality about the ethics of warding a young child... I can dig it up if you like!
(in case you're looking for some interesting reading, Children Who Remember Previous Lives: A Question of Reincarnation
and Children's Past Lives: How Past Life Memories Affect Your Child
are both really interesting books. They were a big help when it came to figuring out dd2's behavior! Especially the Past Lives book by Bowman since she addresses the topic more as a parent from a "how to help my child" angle.)