I highly recommend to
(at least to praise him, who single-handedly cracked the last two easter egg secrets, where others were stuck)
Don't rob yourself of this pleasure!

https://steamproxy.net/sharedfiles/filedetails/?id=3508170387
 
If you are still confused after this or not inspired enough to conduct your own search,

you'll find the illustrated guide below, in the next section
 
 

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In the main room of Jerome's apartment (5th floor, the door on the right), there is a cabinet with a suspicious white panel and some game cartridges inside, behind the glass.

 
This is a reference to a trick in Super Mario Bros 3 where you can hold Down on the white block for a few seconds to drop behind all obstacles

 
To solve this, you need to move cursor over the panel, then press and hold left mouse button for a few seconds.

 
You'll hear a kind of Mario game sound and cursor will appear behind the panel, "inside" the glass case

 
Click on the cartridges to see what was locked from you


You'll hear special "easter egg" sound

Congratulations!

That completes the third easter egg search.
 
 

https://steamproxy.net/sharedfiles/filedetails/?id=3504190844
https://steamproxy.net/sharedfiles/filedetails/?id=3505322603
https://steamproxy.net/sharedfiles/filedetails/?id=3508341057