For this and all of the other intel achievements I recommend purchasing the Canine unit upgrade in the security wing. Once you have this upgrade you will receive a corresponding perk called Intel discovery which automatically activates. This perk will reveal all of the intel that you have not discovered yet in a zone on the map once you complete all of the side missions and encounters in that particular zone.
To clarify it does this zone by zone. For example once you have completed all the side missions and encounters in the Chelsea zone all of the undiscovered intel in Chelsea will then appear on the map. However the undiscovered intel in Pennsylvania plaza and the rest of the map will not until you complete the side missions and encounters in those zones.
Also bear in mind that if a side mission starts in one zone but travels into another the side mission is registered to the zone in which it starts. Therefore if the mission has multiple parts which take you into another zone the mission is not considered complete until all phases are complete and therefore the intel will not unlock even though the map shows that all side missions for that zone are complete. This is important later as one of the side missions in Murray Hill or Turtle Bay has caused confusion because of this.