Dead Space: Extraction

The secret weapons store is found during Chapter 2.

Near to the end of the Chapter you will enter the P-Sec headquarters. You will head up an elevator & eventually enter the room where you started the Chapter in, be careful here because it is also the first time you meet the necromorphs. Fend off the necromorphs and you will meet a girl named Lexine. Once she is following you, you will head down the elevator you just came up in. Upon exiting you will have the opportunity to look around, look to the left to spot a purple light on the wall. This is a fuse box, quickly try and shoot it to open up the secret weapons store and earn this trophy.

If you are too slow and happen to miss the shot then you will automatically be moved on through the Chapter. Therefore you will have to replay the Chapter to have another shot at getting this trophy.