My mother-in-law is a dentist. When she heard I always brush my teeth but rarely floss, she told me flossing is more important than brushing your teeth. For once I didn't get into the science of things, but still I accept that getting old bits of food and other junk away from between my teeth can only be a positive thing. It runs counter to my sceptic nature, I like to question everything, but I'm slowly starting to…
So although dentists could notice if you have good dental hygiene I'm not sure they can spot easily if you floss or not. It might just mean that in US good dental hygiene means that you floss, but converse might not be true.