Weird marks on Brassia leaf

[camille1585]

When I went home 10 days for the holidays I left my orchids in the care of a friend. Being gone for such a short time, I was quite surprised to see a leaf on the Brassia in such poor shape. The damage looks very much like spidermite damage, but there's not an insect in sight, and it's the only affected leaf. And now the leaf is starting to roll itself up at the top, I guess it's drying out.

It's not from the sun, the windowsill it was on faces north. I thought it might have been cold damage from touching the glass (had to cut off some leaves on my Max tenuifolia), but my friend said that it wasn't touching.

Other than disease, the only thing I can think of is that since the windowsill is over a radiator, maybe the leaf was sticking out over the warm dry air that was coming up and started to fry. Is the damage consistent with that, or is it something else?