It didn’t really stand out too much to me, compared to SKB sticking in my craw. During the movie I assumed there was a mystical reason R2 woke up when he did. (Did Luke reach out and wake him back up? Did he detect Rey?) And I still suspect the lame-o answer of “he just then overheard they were looking for a map that he happened to have from the Death Star” might be a cover to preserve a future plot development. I also assumed Luke left the remainder of the map with R2, with contingencies for letting people find him.