Read the Cave Jonhson's slide 1 in the Conference Room :
You will need to spot 5 objects that claim to be different when clicking on them.
One in each zone of the museum. You can see the zones in the Security Room on the wall map.

After finding one or more items, you will be able to enter the Security Room.
On the wall map, click the code of the object that you found (a code list is in the conference room).
Then click the zone where you found it; if it matches, you will unlock the door for the Safe room (You need to report all the 5 items to fully unlock the door.)

Those objects are sometimes difficult to find, or to reach.
If needed read the following clues:

- In the Reception : Above the stairs, have a look at the plants.
- In the Grand Hall : Search at the Café Baltic.
- In the Room 102 : Before taking the elevator, look around you...
- In the Room 103 : You will first need to solve all the puzzles in that room to reach this item. (See the Room 103 Spoiler for the full steps.)
- On the Balcony : Myster X is in the chest locked by a padlock. (See the Balcony Spoiler for the full steps.)