This page in the documentation, Using custom attributes in menus and taxonomy archives indicates
Custom attribute slugs are prefixed with ‘pa_’, so an attribute called ‘size’ would be ‘pa_size’
However, the behavior I appear to be seeing is global (i.e. taxonomy) based attributes are prefixed with pa_
and custom (i.e. local) attributes are not, which I believe is the opposite of what the documentation says.
For clarification, I'm referring to a custom (i.e. local) attribute as one I add to a specific product using the Custom Product Attribute Add button.