Just noticed these spots appearing on some of the leaves of my Maori potatoes being grown in a jute coffee sack. Plant otherwise looks healthy, and i don't see any insects. It's not new or old growth affected.
Looks almost as if the leaves have had something dropped on them, but the likeliest explanation is, unfortunately, some type of potato virus - if the lower leaves start to go completely yellow, or other symptoms appear, you'll know its a virus. However, sometimes there isn't that much to see on the leaves, but the tubers will be affected in some way. There are a lot of viruses which affect potatoes and many haven't even got proper names, being listed as things like 'potato virus Y' or whatever, but there is some information in the link below regarding viral infection