I will say it again -- probably at LEAST 50% of cavaliers will get SM over their lifetime. This is NOT 'someone else's problem' -- everyone of us likely has at least a 1 in 2 chance of our dog having syrinxes, and researchers believe about 95% of all cavaliers have the skull malformation which can alone cause pain and problems (so much so that in humans most people get CHiari -- the malformation -- and all its symptoms before they go on to develop SM). So every single owner should be thinking OMG about SM -- it is devastating the breed and there are prominent geneticists who do not think the breed has a very good chance of survival.
That said most dogs do not exhibit symptoms. Is this because they don't feel pain? Yes, many probably just adjust to it and maybe only get minor skin sensations or none at all just as some people with syrinxes never have symtpoms. But I am not so sure about many dogs thought to not have SM (and is why I think there's a reason every single vet I know says cavaliers are particularly 'wimpy' when it comes to injections... in their neck... gee I wonder why that might be?). There is very clear evidence that the dogs will accommodate the pain over time (even extreme pain) and most animals hide pain very effectively. My own vet says Leo has a very high pain threshold, from treating him for other conditions. When he occasionally gets pain on his side and his limbs from SM, I usually cannot tell even by handling him except by watching his body language. A neurologist -- or my vet -- would pick up his reactions as a pain reaction but quite honestly until recently, and despite having dealt with this for some time, I would not. I understand stress signals such as lip-licking much better now and can see that he is actually in pain when I touch him in certain places. I have realised I have likely been greatly underestimating the amount of daily pain he lives with (for most dogs, pain has to be pretty extreme before it causes them to stop playing, walking, or interacting with their owners -- just ask any vet). I will have Leo in for another MRI and neuro exam to assess his neurological response. Owners and even vets cannot really do this with any accuracy.
The difficulty with SM is that some behaviours are exaggerations of normal behaviours. If you have doubts about what is normal, then talk to a vet or a neurologist. or start to watch other dogs or talk to other dog owners (though taht said, not one of my friends with other breeds of dogs would notice what is abnormal about Leo's bahviour but I see it immediately -- the degree of scratching, the hair loss on his ears, the night scratching, the bunny hopping). An astute owner will start to see a clear difference between occasional face rubbing (common) and rubbing on the sofa (common) and face rubbing because of facial discomfort, scratching due to discomfort, and so on.
You really need the sound to understand Chester's video as the voice over talks about why these behaviours are specific to Chester.