I saw this series of 4 circles in the schematic of an Arduino board. Similar circles appear in other Arduino schematics, though I don't think they are Arduino specific (which is why I post here rather than on the Arduino forum).
What do they mean?
OK, I found an Eagle schematic file of this, and I checked the properties, like jwsc suggested. They seem to represent fiducials.
Comment: fiducials don't belong on a schematic, they're purely PCB-related. You don't place mounting holes on a schematic either. When they appear on a schematic they will most likely show up in the BOM as well.