You may have spilt nuted water on a leaf? The very defined border of burnt patch makes me think it ain't heat stress considering that HPS hue in the background and the fact that heat comes before too much light in HPS terms.
I think you spilt some "hot" water on the leaf......
I've had the exact same thing on some clones I put outside. It rained, and when the sum came out it burned the leaves just like that. Almost lost a few...