So in the campaign, one of these guys is hidden in each mission.
Find all of them in a single playthrough and you'll get a slightly extended ending scene!
NOTE: If you find one of these, scan it! If you do that successfully but then fail the mission, you will need to find that snail again unless you reached a checkpoint after you found it.
Use this thread to assist others with the hunt. Eventually I'll post a guide, but I want to give players a chance to search on their own first!