The team never really gave it much thought, opting to keep moving forward on the project. When I was asked about it my answer was, "A view is assigned/using it." The response was, "No we've fixed all the views already but it won't go away!". We checked again using a view list and sure enough the rogue value wasn't being used.
That's when it occurred to me that a view parameter is stored in View Templates. In fact a view template is the reason the value keeps popping up every now and then in a view's properties. We cleaned template after template, the project had quite a lot of them. Moral of the story? Remember View Templates! They capture view related data too!
A video might tell this better so check it out. I've also embedded it here to listen and watch.