I think it is just normal wear and tear. Perhaps someone in your house, or even yourself, may have accidentally bumped into the plant. Or perhaps the edge of the pot was a bit rough or had fertilizer salt on it that the leaf rubbed against. The position of the cracks on the leaf seems to have been resting on the edge of the pot. If the leaf is in a vital position on the plant, you can leave it alone. It is not diseased and the damage won't spread. Joanne

I agree with Joanne. This is some kind of mechanical damage not an environmental one.

Linda