During the main quest Seeds of the Past, Aloy will scour the Test Station Elm with her newest companion Alva. Together, the two brainiacs must solve the puzzles inside to reach the information they are looking for. One piece, in particular, can be especially elusive, the maintenance key.
Where the Maintenance Bypass Key is hiding

After entering the Greenhouse through a vent, players will have a couple of levels of this inner chamber to explore. First head down to the lowest floor, and head to the far right corner where a battery storage container is. Pull this out of the wall and check in the space behind. This is where the maintenance bypass key will be.
For more Horizon Forbidden West content, be sure to check out All Black Box Locations in Horizon Forbidden West on GameTips.PRO.