This is obtained on level 2-1. It is Miss Ions' mission. It is simple enough, once you find her, she will give you a key and ask you to get her jewels from the safe. You then need to find the safe and search through it for all her jewels. When you return to her you will get the trophy.
The safe room will appear on the map as an exclamation mark (!). No point heading to it until you find her as you can not unlock it without the key she gives you.
Once you have spoken to her, a jewel icon with appear next to the map in the bottom left. You need to get this to zero jewels before returning to her.