My son will be 4 the first of May, and he starts talking about things that I didn't even realize he noticed. Like things he saw on the TV or over-heard someone talking about. Something might have just triggered the memory at that moment and made him/her want to talk about it. It always amazes me how much my boys (soon to be 6 and 4) pick-up and remember. If the experience seemed inappropriate or troubling you may want to bring up the topic in private and discuss it with the child. Otherwise, I don't think it's really that unusual.
God Bless J. <><