I would say if comsec has sold without your permission you would have owed them money through other transactions. but you would have been called prior or texted to say you need to settle an account. They do have the power to do that to balance the books.
i would be checking your confirmed buy/sells and seeing why such a thing happened, if you cannot see why then a call would be your next option. from there is all depends on what was said over the phone.
