In the Hand Jive scene right before Cha Cha cuts in, whenever Danny was on Sandy's left, his handkerchief was also on his left front pocket. Whenever Danny was on Sandy's right, his handkerchief would also be on his right front pocket. The handkerchief keeps switching sides!
You are incorrect. I watched the scene several times from beginning to end, and there is not pocket on the opposite side of Danny's jacket, much less a dancing handkerchief.