Battlefield Hardline

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.

After planting the bug upstairs in the mansion, make your way downstairs, arresting bad guys and whatnot. There will be two guys sitting down on the main floor. Scan them and one of the should be Cameron Briller, one of your suspects. After taking him down, search his body and a safe combination(which is also a piece of evidence) will be yours. Now make your way to the gym. There will be two bad guys there. Take care of them. Once you're done, you should see a latch on the right side of the TV. Interact with it and a secret door will open, revealing the hidden room.

Also, there will be another piece of evidence on the corkboard in the hidden room.

(Sorry if this explanation isn't the best. Explaining things was never a strong point for me.)

I will try and have a more thorough explanation to the solution if people would like it.

After planting the bug upstairs when you make your way down stairs there are 2 guys sitting down. Arrest them first after that make your way to the gym where the secret room is.

See the video for where the secret room is.

Here is a video guide with English commentary for the "Hollywood Hideaway" achievement!