Locations in add-ons will be revealed by this perk, but only if it was taken after the add-on was installed. If the add-on is installed after this perk is taken, those locations remain hidden.
The two locations added to the main map by the Lonesome Road, namely Long 15 & Dry Wells, both appear when this perk is taken. However, they are only accessible depending on how the Courier chooses to complete Lonesome Road.
Sometimes in FO3 and New Vegas when you take the perk no location will be revealed. [verified]
In New Vegas, you can level up high enough to choose the Explorer perk before leaving Goodsprings for the first time by using the infinite XP exploit with Doc Mitchell or Chet. Then, when leaving the town you can rebuild your character. The Explorer perk won't be available because you've already taken it, however, all the markers will remain on your map. You can then choose any perk instead of having to choose the Explorer perk. [verified]