I'm not sure how to explain it beyond the title - basically, established settlements give an "under attack" notice... but when I go to the data tab, select the mission, and click "go to", it doesn't bring me to the settlement... when I eventually find it on the map, it is now an "undiscovered location". This has happened... at least a dozen times so far this playthrough? It seems to have started after the point where Jake was taken by the Gunners if memory serves.