Cats can hear a whole range of sounds that we humans cannot. They can hear sounds up to around 60 kilohertz while human range peaks at about 20 kilohertz. A dog's range peaks at 45 kilohertz. This explains how cats can hear high pitched mouse sounds while we hear nothing.
Max used to make what I called 'silent meows', where he opened his mouth like he did when he meowed but no noise came out - well, no noise that I could hear. Cats have the option to communicate in a frequency range we can't naturally detect and yet they often choose to use a range we can hear, how kind of them!
It's weird to think that my cats could be carrying on a conversation between themselves right in front of me with sounds only they can hear. If that's possible, maybe it's possible too that they can see things that are beyond my scope of vision. Maybe Nahum really is seeing ghosties when he gazes up and around in the air like he is intently watching invisible objects move about.
(information gleaned from The Body Language and Emotion of Cats
by Myrna M. Milani, DVM and Louisiana State University School of Veterinary Medicine, Deafness in Dogs & Cats website.)