A true undead's senses are simply that much more superior to anything else, I believe that most of my readers are already familiar with the concept of the undead synesthesia, whilst similar to what we, as living, experience when we speak of synesthesia, for the dead, it is much different.
It is not a mere impression, not only does a pure undead possess the five basic senses, which can not be impaired through natural means, if at all, as well as armed with 'dark vision' and 'darkness awareness', they can feel everything about someone based on just one sense.
By this, I mean that if a pure undead heard me moving around, it would be able to 'see', 'smell', 'feel' and even 'taste' me, it is as though all of their senses were merged into a singular thing.