Find the auction bidder and hack his phone. The security router you must hack is found way back in Brandon Docks near your hideout to the north. The security layer is quite easy to break through so you won't have troubles finding the briefcase.
Find the next bidder in the walkway between the two buildings as marked in the map. Next, travel all the way down to Parker Square to find the building where the briefcase is located. Find the vulnerable security router and hack it. Scan the briefcase to complete this activity.