In Harrumat diamond mine, that key is too hidden given the low reward locked behind it. I looked for 30 mins with a detect key enchantment on, couldn't find it and googled where it was as there is no hint other than it's 'nearby' which indicates it's in the cave somewhere. There's almost no way that someone would find it under the planks in a chest almost the same colour as the planks and floor as the box is only visible from a very specific angle.  I had a detect key enchantment on and because it's in a lockbox it doesn't show up. perhaps just make the key loose on the floor so if someone takes the effort to get an detect key effect, they can find it quickly.