As you said, it's just a brown spot. Photos of the rest of the plant would add context. Maybe a leaf rubbed against another leaf? Or, some injury long ago, as leaf emerged. (I had a cricket, or something, munch on emerging leaves. When those leaves were older, they looked like a carnivore had taken a bite. The way an early injury can enlarge, it could have been something minor a long time ago. If that's the only one you see, and if the plant looks healthy overall, I wouldn't worry about it.
I had a thought, could these be caused by heat? Been hot as hell this year, just got out of the 90's.