This takes place on Episode 7: Glass Houses and can be done after reaching the top floor of the house and then watching the cinematic.
After the cinematic, you will regain control of your character, and you'll want to make your way downstairs arresting enemies or using the taser. Scan the two enemies in the living area on the main floor, and one will be Cameron Briller. Make them "freeze" and arrest both of these enemies, and then search the suspect's body in order to find a collectible. Now make your way to the gym room which is located on the opposite side of the house from the garage. You'll find two enemies in here that you'll also want to take out.
Now that you're inside the gym, you'll want to look on the right hand side wall, next to the tv, and you should see a small indent on the wall panels. Approach to "Open Panel", which will reveal a keypad. Interact with the keypad to open the secret door, and there's the hidden room! Inside you'll find a collectible as well.