It appears to be mildew, which means there is moisture coming in that area from somewhere. Time to check out the roof really good.

Mold or mildew has no concept of what the size of a sheet of plywood is, so it wouldn't stop perfectly once it gets to the edge of a sheet. So either that specific sheet was already wet sitting in the pile before putting up on the roof, all the plywood was moist under there and they replaced a sheet as shown in the tiny perspective of the picture, or an open bath exhaust fan duct stopped in the attic under that specific sheet (and replaced the sheet above it). But with relatively new trusses, I would suspect no sheathing has ever been replaced so the first option was probably the case.

I agree with 3onthetree. My first thought was that that sheet of plywood probably looked like that when it was first installed. You could always buy a lumber moisture meter and see if the moisture content is any higher for that sheet than the others. But I doubt it.