For other fields, we have values like 'none' or 'unknown' to differentiate a blank field from an actually unknown or non-existent bit of information.

Does it make sense to have one for area?

Example:
You can say that a recording was made at place:[unknown]. and leave area blank
You can say that a recording was made at place[unknown], area:UK
So, shouldn't it be possible to say that a recording was made at place:[unknown], area:[unknown]