The HCU is under the pylon on left side underneath the cowling, it isn't visible externally so can't detect incorrect HCU config during walk-around.
There is a streamer-free pin that lives up there and stays with the aircraft. that is what the engineers used to lock out the reversers and forgot to remove.
Very sloppy maintenance. If I'm not mistaken, landing performance is predicated on braking action with nil reverser credit? (I'm not a pilot).