I am of the school of thought that most animals have a sixth sense about other living things, including human beings. I love all animals and have owned many different types of pets. I have come in contact with lots of animals and have seen nervousness in certain animals when being held or touched by one person that vanishes when held by another. On a recent visit to the Dallas World Aquarium, there was a bird on the handrail in the rainforest area. A gentleman in front of me kept attempting to pet this bird and it kept ducking its head and inching away. I reached out to pet it and it actually leaned into my touch.
With regard to our pet snakes, here's something that I've discussed with my husband yet have never seen it addressed in this forum. Snakes do not have ears and don't "hear" the way other animals do. However, they are aware of vibrations. Our voices reverberate from our larynx down into our chest and out into the air when we speak. Different voices carry different patterns of vibration. I believe a snake can "recognize" it's owner's voice by its distinct vibratory pattern, i.e. frequency.
Call this far-fetched, but that's my opinion. I truly believe that Alice knows me by my scent, by sight, by the method in which I handle him, and by the "sound" of my voice.
Not necessarily scientific, but definitely hypothetical!
Robbin