You can start looking for them as of today, because private matches are now live as we close in on a week before the new Rise of Iron expansion is released. Buried in each map we’ve tried thus far is a dead Ghost, which when revived also unlocks a new Grimoire card, adding to your understanding of Destiny lore.
Destiny players are already discovering the Ghosts and uploading their finds to the Destiny subreddit, so avoid those posts (and the inevitable YouTube guides that will pop up shortly) if you want to make these discoveries on your own.
In addition to new Ghost hunts, private Crucible matches in Destiny offer 15 game types including Inferno (no radar) and Mayhem (quick ability recharge rates, no respawn delay, more ammo drops) variants. There are 26 maps—every map released for Destiny to date—to choose between, six different score and time limits respective, and the ability to switch on Light advantages for your own private Iron Banner games.