I read in a newspaper that we can't predict the trajectory of a hypersonic missile and that this property renders the missile undetectable.

However, what I could not understand is why can't we predict it's trajectory? What factors do we have to look at for predicting the trajectory of such high speed missiles? Is this feature associated with its speed? I know that there would be forces like the thrust from propulsion, gravity, and the drag force. Is there anything else affecting the trajectory?