After completing a story mission or two, Yuri will have you meet her at the Police Station indicated by a waypoint on your map. When you head there, a cutscene will play out where she informs you that the towers that provide radar all over the city have gone down and she needs you to get them working again.
After repairing the first tower, Peter will automatically detect the first backpack indicated by a bright green symbol on the map. Swing over to it and press Triangle and you will collect the bag.
After this, the remaining backpacks will be shown on the map. If you have unlocked the towers in each area the bag will show up on the mini map as you get close. Alternately, you can click R3 and the bags will emit a green beam of light in the sky giving you a nice indicator where they are hidden.
As soon as you press triangle on the 5th bag, this trophy will unlock.