The flashing and beeping is just a reminder that you've left your handbrake on. It's in addition to the normal sold light that comes on when you're stationary.
I've had this occasionally when I've not disengaged the handbrake completely, and it's been left on the last notch. It's not enough to have any braking effect, but enough to set the alarm off. So the first thing to check is that the handbrake is definitely off. After that it might be a switch fault.