So true. They definitely seem to have their "off" days too, when they just want to be left alone. The tell-tale signs are the same, but every snake is a little different. It's just a matter of learning your animal.
My King, Django, for example will "push" my hand away if he doesn't want to be handled. By that I mean he will thrash the part of his body where my hand is when I touch him. It still impresses me how strong such a slender snake can be. Tyson, my BP will do this on occasion, but much more gently. Juice, my Borneo STP, is more of a hisser. But it's always pretty mild, he hasn't impressed me yet, hahaha.
Anyway, since they can't verbally tell us they want to be left alone, they show us the only way they can. Depending on the species and sometimes the individual snake, the number of "warnings" will vary, so it's best for us to do our homework and learn the animals.