All credit goes to Reddit user 'windkitz'.
Here's the Reddit post for full context: https://www.reddit.com/r/redfall/comments/1381k4w/comment/jj
There is a map for each location and a series of screenshots giving further context to where each grave lock is. The numbers on the maps match the number under each image in the series.
Once you leave the first area of the game, Redfall Commons, you cannot return until new game plus. Similarly, once you defeat the final boss of the game you are forced into new game plus and cannot return to Burial Point until your next playthrough.
EDIT: Grave locks are attached to your hero, so they carry over between playthroughs and also carry across on your hero if you join someone else's game save.
Redfall Commons has 45 grave locks.
There is one mission here that if you don't get the grave lock during the mission you will not be able to access it. The mission is a main mission called 'A Grave Situation: Reward' and its location on the map is number 33.
In this mission you go collect a reward from Dr. Hunt's car. The grave lock is in his garage (which is only open during this mission).
Images link: https://imgur.com/a/e39IjBa
Burial Point has 55 grave locks.
There are no missions that passing will mean you can no longer access a grave lock like with Dr Hunt's garage. However there are 2 grave locks you cannot access until you complete certain missions.
Number 21: You can't access this until you complete 'The Key To Wellness' main mission and pick up Miss Whisper's Key. Once you have the key you can unlock the basement area of the building. (There is a side mission down there.)
Number 55: You must complete part of the Blackwood Sisters Puzzle (Sisterhood Achievement) and gain access to Hesters Room to get this grave lock.
Images link: https://imgur.com/a/aHtGdYs
Once you have all 100 grave locks you receive a bonus of where your ultimate power will automatically regenerate the first 25%.
I don't usually post guides, this is just a placeholder until the author of a guide comes along.
EDIT: I've decided to keep this up here, as if you're anything like me, sometimes you like a map guide so you can get them while you do other missions and play through the game.