MOST WANTED: Gunrunner
Becomes available after destroying the first weapon cache during Mission 09 - Interrogate Penguin.
Objective: Find and destroy all of Penguin's weapon caches
Objective: Awaiting Penguin weapons van location update from Nightwing
After finding out about Penguin's operation from Nightwing, tear across town to find the Penguin goon van waiting for you. If you approach in the Batmobile, Batman will tell you to approach on foot instead. Climb a nearby building and tag the van with the Disruptor.
Once the van is tagged, walk up and freak them out by knocking on the door (seeing Batman's grim and scratched up face would be a good incentive to slam the accelerator). Keep up with them using your Batpowers of gliding/ flying and they will lead you to their hideout.
When they stop, walk up to the building and Batman will inform Nightwing about Penguin's warehouse location. Stroll inside the building and use the hacking tool to open the door opposite the two sentry turrets.
Go back outside and summon the Batmobile, using it to destroy the turrets. Inside you will need to use a lever to open the roof cover to allow you to drop in on the enemies and say hello. Batman will make this comment too while you stand on the rooftop. Take out the room full of enemies (Nightwing will help, allowing you to use team takedowns). Inside the vault is another cache that you can use your Explosive Gel on, close the door, and boom. Job done.
Note: This and the following gunrunner activities can become glitched. They may only unlock after completing Mission 23 - Return to Panessa Studios (They did in my game anyway).