It could be this, mentioned in the schematic: “Spark Gaps - Case: Since the case has gaps in it we expect [electrostatic discharge] to worm it’s way in via cre

It could be this, mentioned in the schematic: “Spark Gaps - Case: Since the case has gaps in it we expect [electrostatic discharge] to worm it’s way in via creepage and perhaps other ways. To protect the board from this eventuality, we place spark gaps along the edges.” https://github.com/ploopyco/mouse/blob/master/hardware/Electronics/Schematics/Base-R1_003.pdf https://github.com/ploopyco/mouse/blob/master/hardware/Elect... So technically a via fence, but not quite as sophisticated.