As of right now, in the game there is no way to send out units that cannot be seen by most enemies. A strategy thats as old as time itself is that of having hidden men to report enemy movement. This is a MUST for 0 AD.
Hello, I'm so happy to see a Viking Civ in the works! Has anybody here been following the History Channel's "Vikings"? I do believe you will find a lot of inspiration from it! http://www.history.com/shows/vikings