There are two Hidden Vault Symbols in the Southern Shelf area.
The first one can be found during the "Shielded Favors" mission. After taking the elevator to the top building, buy your shield and then walk through the other side. You'll find yourself on another balcony with rocks you can climb on to your right. Stand on those and then turn right so you are looking along the side of the building. Climb up the piece of roof here and at the end, jump onto the crane. From the crane, jump over to the roof and then walk across. You'll see a wire mesh under you. Drop onto that, turn around and you'll find a small balcony. On the floor of the Balcony will be this Hidden Vault Symbol.