I think you are onto something with the clear housing.
When really protecting the real secret information, there are organisation with cables that don't have sheathing, port plugs that are transparent and cable trays are visible in the open and the end user devices are stored in safes when not in use or on always on person.
This idea seems like a nice easy option, the attacker could just as easily swap out the end user device (keyboard) for a custom compromised one.. makes you wonder if letting them think the cable trick has worked was the ideal way of showing their hand in a more detectable manner..