While searching for the collectibles hidden inside the Gotham Bridge (Datapacks 11 and 12) you will have to visit a few rooms inside the main structure of the bridge (e.g. the boiler room and the train station). You visit them for the first time during the Warn Gordon about the bombs an Locate Firefly's bombs main missions. By way of reminder, the entrance into the bridge is in the location shown in the above screenshots, i.e. in the Western side of the central structure.