Small, round grey spots/indentations on leaves

[ckanowsk]

Hello,

I've had this orchid for about 7 months, and it's usually done pretty well. In the last few weeks one leaf has been showing these very small, grey spots. They're almost indentations in the leaf, and once they appear, they don't grow. There's no yellow halo. I've included pics for more detail.

Any thoughts as to what this might be? Thanks very much for your help, everyone!