*Story Related, hard to miss although it makes you change back to Batman so find a Catwoman switch icon if not playing story select.
It's the final mission for Catwoman, Two Face has stolen your goods so you have to get them back. There's 16 to collect, you'll see a Gem icon appear above the Two Face Thugs ( while in Thief Vision similar to Batman's Detective Mode). You simply need to beat them up without dying and the trophy is yours.
Note: Catwoman can be much louder than Batman when sneaking up or traversing around so keep in mind where your enemies are, they can kill you quite quickly with their assault rifles.