Duke Nukem Forever

The secret closet is located in the end of "The Forkstop: Part 2". After Duke has told you to stop forking around, you'll enter a big dark room with a second forklift in it. This one is hoisted as well as the first one, but you can't lower it down to the ground. That's because there's no power in the room. Continue as you should to get further into the game and you'll come to a switch which turns on the power for the room you just were in. Turn it on and go back to the room which is now lit up. As you turned the power on, Pigcops have entered the place. Kill them all until there's not more than pieces of them. Once every alien is dead, walk up to the control panel next to the forklift and press . Then hold to lower it down. Once it's on the ground floor, enter it with and drive it to the wall to the right and raise the forks all the way up using . Now, park it with the forks against the wall. Get out of the forklift and jump up on the back of it, all the way up to the top of the carriage, like if you were walking up some stairs. You will now be standing beside a balcony. Jump to the small ledge and jump over the railings. You will see a dark corridor. Turn on your Duke Vision and enter it, and you will stumble across a barrel with a heart on it. Grab the barrel back out from the corridor and jump down from the balcony. On the opposite side of the room is another balcony, and to the right of it is a platform with a green button beside it. Press the button with to lower down the platform. Jump up on the safety poles around the platform to get up on it. You're now able to get up to the balcony. Once you're up you'll see a corridor which splits up in two ways. To the right is an elevator, and to the left is a closed door. Walk towards the closed door to the left and you'll see a pink heart on the floor to the right of the door. Put down the barrel and the door will open. In there you'll find lots of weapons and other items, but you will also get to hear the beautiful sound of a trophy unlock.

Here's a video guide of the trophy:

Credit goes to PowerPyx.