You'll get some kind of popup message I believe. Just like it says the same thing about sharing it when you find a hunting spot for the white whale.
The social chests specifically show up as a blue colored treasure chest on the map. I just found one on Abaco island near Nassau. --- Xbox GT: AGE Sam Sneak "Like an abandoned school, I have no principles."
you dont need any one on your friend list to find social chest i know none of my friends have ac 4 and i find them all the time with the message its sharing its location which i know its nobody --- 360/PS-Neohaven fc: 5000-3029-2245