The Gilded Grasshopper

The Gilded Grasshopper is a side quest in Fallout 4.

Detailed walkthrough

 * After the Sole Survivor reads the file about Marty Bullfinch in Nick Valentine's office, the mission of retrieving him as a missing person, can be obtained.
 * Going to Faneuil Hall and killing all the super mutants (at least fourteen) on all floors, will lead to the roof, where the dead body of Marty Bullfinch and the gilded grasshopper can be found.
 * The gilded grasshopper is atop the golden dome, next to the ruined belltower.
 * Reading the note inside, Food for the grasshopper, points the Sole Survivor to the grave of Shem Drowne near Pickman Gallery containing the skeleton of Shem Drowne, his lootable skull and sword, a gold bar, two silver bars, and three copper bars.

Behind the scenes
The Gilded Grasshopper is evidently based on the Grasshopper weather vane on top of today's Faneuil Hall in Boston, created by Shem Drowne.

Bug

 * If you completed the quest before activating the case file. The detective file will be locked and you cannot continue the next file.
 * When digging up the grave, the game may freeze or crash.