Inconspicuous Note

The first Inconspicuous Note is found at on a table upstairs in Curiosities & Moore in Dalaran above the Broken Isles. It holds the first clue for obtaining the Lucid Nightmare. See Riddle of the Lucid Nightmare for more information and the whole list.

Solution
2104059 is an anagram of 2009-4-15, the launch date of Patch 3.1.0 in Europe. The key feature of Patch 3.1 was the Ulduar raid.

In the Scrapyard where the XT-002 Deconstructor is encountered, find a Rusty Lever on the scrap heap in the north-western corner of the room. Activating it will spawn a large field of 20 x 20 Scrapyard Lights in the middle of the yard. The lights must be activated to match the sigil of Mimiron (an eight-spoked cogwheel), seen here or here. Mimiron's sigil is the only rotationally-symmetric icon.

Once this is done, all of the Scrapyard Lights will despawn and a new Inconspicuous Note will spawn in the middle with the next clue.