There are 72 unique points required for this achievement. You'll want to explore every area thoroughly as it seems some buildings in an area may count as a location by themselves, or what appears to be one area may be separate into a few (such as the interior of the hotel). To see if you're on track, you should have 18 at the end of Act 1, 45 at the end of Act 2 and then 57 at the end of Act 3. Make sure you have 57/72 locations before the "point of no return" (the game warns you when this occurs) as there are 15 locations to find in the prison before the game ends