I recently worked for a customer that showed me that all their network cables are visible. Indeed, cables were never drawn inside walls, conduits or trunks. Instead, they were "hung" on poles close to the ceiling. If a cable needed to pass through a wall, the hole was way larger than required, allowing a lot of light to pass around the cable. These measure were allegedly taken to make it very difficult for anyone to do network wiretapping inside their premises, without this being immediately noticed.
Does anyone have a clue what security standard they followed?
My understanding is that they were trying to avoid this kind of attack.