The lock isn't to a chest of gold, but to a "time capsule" room in an old library that contains letters written by your ancestors to you.
Every time you go back to that physical spot, the door is gone. But in the video, it remains, slightly ajar, with a soft golden light spilling out.
The story follows your character retracing their steps across the city shown in the video to find the lock that matches the key.
The video captures a fleeting moment where a stranger drops a small, ornate key into your bag without you noticing at the time.