Did you die / wipe? If so that resets the ghosts. I think you can just go back and pick up the first without going to orbit, but depending on how well you know VoG you may be better just restarting the mission. Otherwise, and not trying to be funny, are you looking in the right place? The second is the hardest to find IMO. This is easiest doing in Fireteam to avoid the wipe issue. Whoever picks up the first ghost has to pick up them all, but you all get the grimoire cards and the cult ghost at the end.