"Nanu" is a sort of onomatopoetic expression for surprise. It's almost always put infront of the sentence to indicate that you took note of something you find especially peculiar. It's often followed by a rhetorical question about the matter, but doesn't have to be. A very typical phrase would be "Nanu, was ist denn das?" or "Nanu, das ist ja komisch.". I hope that helps, if I need to clarify something, I'd be happy to.