Besides playing full memories of Black and Red, Get Even also allows you to search hidden memories. They are small rooms in which additional interactive items are stored. These hidden rooms can be found when you replay Black's memories. Those places can be recognized by metal doors or gates near which you can find an electronic lock that requires you to enter a four-digit code, an example of a such place can be seen in the picture above. The list of hidden locations can be found in the following pages of this guide.
Door combinations can't be found in areas so don't look for them. You must make some preparations before those hidden rooms become available to you. In order to get access to a secret area you must find 100% of tracks in a given memory. When you return to the evidence room you can look closer at the evidence board of a given level. A four-digit code will appear in the bottom left corner of the table (an example is presented in the picture above). Remember the code, replay a memory (interact with the picture in the bottom right corner of the board) and reach the place with locked door. Then, you can enter the code.
Warning - Do not try to enter a code before you find all tracks. Even if the code is correct you won't be able to enter inside until you find all tracks. As a help for locating other evidence you can use empty silhouettes in a board that look like, e.g. A4 notes, photos, small business cards or big posters. In addition to that some tracks are connected by a line. This can help you in guessing which part of a memory can have those tracks.
Warning - By unlocking the first hidden area you get Cogito Ergo Sum achievement. By unlocking all of them you get another one, Gnosis.