It's the one that says that you have to go find Freeze's stolen weapons. Can I just go to the Joker's hideout and start looking? Or do I have to activate it somehow?
just go in there and kick a$$, grab the loot and be out. Its kind of tricky to find, but just snoop around
If you see the side mission listed then that's all the activation that is needed. Just go get it. That's it!