Green on your maps means you haven't explored there yet. Missing one location will be a huge pain later when you have to check every location again. Don't let co-op partners run through quests without you. I've listed a lot of locations below as 'Unmissable.' Obviously this is assuming that you yourself do the quests and complete the story. Do all of the quests the prisoners give you and you'll find every location in the prison. There are 15 locations at the prison that can easily be found by doing all of the quests.

You need 57 locations before going to the prison (Act IV).

Your achievement tracker will have gone up by one more location. To get it to count twice, once you've been to the gas station and it has counted once, quit out and then reload your game. The gas station in Moresby is a weird buggy thing and counts twice. There are only 71 actual locations in the game. Keep dancing around an area until you find that succulent sweet spot to make it pop. Some places like the Gas Station and Oldtown Bridge may be a bit awkward to get the text to pop up. First thing you need to know is that you need to see the actual red text appear in the bottom-left of the screen for the location to count.

Dead island act 2 map