Oops, posted in wrong thread
It seems that once you have had the board in your view at any point it automatically adds to your map.
I passed one on an online road trip and it was on my map when I looked later on.
Only because you passed it within a certain distance; I think the treshold is about 25 yards between car-detection and board-location for it to automatically appear on your map.
Internet is best treasure map, when needed. So far I’ve used it to find the last 2 fast travel signs. Never woulda found one of the two without it, buried as it was deep in a random forest.
Found all the barn finds without it tho 
B Wald
Sorry about the bump, but another thing to do that I’ve done is pull up a treasure map in Google/Bing images in Xbox Snap, and look at that one on the same screen.