Martin describes the missile arcing gracefully downwards and towards him with a good solid smoke trail. It could have been ballistic, I suppose, and I really do not know what the vapour trail from a partially full drop tank would look like. You are quite correct that Sharkey mistook them for missiles.
It is worth pointing out here that Martin thought he was under attack by several Mirage III's armed with semi-active front hemisphere (Matra 530?) missiles. I believe Tony Penfold called multiple hostiles during the run in which further confused the picture.
ES