Automatic type IRI generation in Playground

I'm curious why with the below configuration, where I've specified a , the
Playground is generating the type "http://json-ld.org/playground/UNKNOWN"
in the expanded and compacted output even though the term "UNKNOWN" is not
defined in the context.

It's as if the playground is partially applying a @vocab for this case.

See:
http://json-ld.org/playground/#/gist/7b4d161af78a9308954918a231b3d76d

I couldn't find a relevant section in the JSON-LD spec on first glance that
said this should be done. Any tips?

Nate