If it's not light burn or heat stress caused by a hot spot then I'm baffled... I think that nute burn would be more systematic and affect more than just a couple leaves in one area.

Here's why I think it's light burn...
[attachment=o253792]

Or heat stress...
[attachment=o253793]

Keep an eye on it, maybe move it to a different spot (if you haven't already) and see if it gets worse. I don't think it'll get better, but if it stays the same then nothing to worry about.