Let's say a broker has a client who had a call option that was out of the money at the close of trading. The client did not elect to exercise the option and instead opted to let it expire worthless. However, in after hours the underlying moved and the option had some value. So the broker could exercise the call option and short an equal amount of shares in the aftermarket to lock in a guaranteed profit. The client would never know. Is the broker allowed to do that?