On my local course, one hole has a wasteland area running along the right of the fairway, it used to have a drainage pond and lots of trees, if you hit there, you were dead. They started cleaning it up, moving out dead trees, clearing out the area. It looks nice now but it's still just as bad to hit there.
One day we had just teed off, I hit right there, and then I noticed the stakes and cord marking the area off, they had replanted some grass in the area where the trees and deadfall were removed. At that moment, the course owners son (The course PGA pro) came up on us and asked if we would check in when we made the turn, I assumed they just wanted to know because there was no one behind us.
He asked how I was hitting, I was having a good start, bogey, par, bogey...then this crappy tee shot into the crap. He said "That's ground under repair, you get relief. Anyone asks, you just tell them a PGA pro told you!"
We laughed, he drove off and I proceeded to take my relief!
So what that tells me, at least at my course, if there is anything planted new, and it's staked off, then it's ground under repair!