Assassin's Creed Unity features 40 sync-point collectible in all, and YouTuber PowerPyx has created a number of videos to help you find them all.
The can only be discovered during the game's co-op missions, and you will unlock one upgrade point for each. collectible you find. Once you have enough points, you can use them to buy new skills and they are required for thee "I Got Skills" trophy or achievement.
By default, they are not marked on the map and the only way to find them on the map is to use eagle vision when in close proximity.
You'll earn two or three skill points for each co-op mission you complete, while the rest of the skills can be obtained from story missions. Only after completing all missions and sync points will you be able to fully unlock and ungrade everything.
You can watch all of the videos below.