Here are several plausible justifications:
Ghosts can only be seen by very few people with a rare psionic ability which happens randomly. There was no way to directly show the ghost to others, so the protagonist had to go find that ghost again and tried collaborating with it, persuading others by getting inaccessible information. For example, asked it to search others' backpacks and pockets and then told them what was inside.
Ghosts should not exist and would immediately vanish once they could be seen or get recorded by other means. This includes leaving a mark in the real world. The protagonist met a special one that can last a bit longer, perhaps due to a tragic death or an unfinished mission. To prove the discovery, the protagonist set up cameras at the same spot and then tried communicating by leaving Y/N question notes for the ghost.
Ghosts can only enter the real world through a specific structure like a portal which doesn't exist for the most part of history. The last time this happened can be one of the historical events when someone reported seeing ghosts. This time it happened in a wasteland where no one went except for the protagonist. So the solution was to bring a whole team to where it happened, and they might run into some crisis involving the portal or got transported to the other side.
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