Saint Chapelle. That must be a sign to tell foreign or new Assassins that Saint Chapelle is their Headquarter. The Templars took Percy Westcreek to see if he could find anything with his Eagle Vision. I guess he founded that.
Some symbols are just there as "easter eggs" or just for being there like: "Oh, I found these symbols... quite interesting. Ok, let's keep murdering". There is a mission in Unity where you have to steal a chalice in Notre Dame, once you find one of them you'll see this. Those symbols indicates where the chalice is. "Hey, I found this chalice and these symbols kinda told me where it was. Ok, let's keep murdering!"
Here is the video. If you notice, it's the same table.