I think the generic issue(s) you’re poking at is how to distinguish (the encoding of) non-existence from not-knowing.
The second issue is encoding errata, i.e. you forgot to tag something or you didn’t know how to tag it properly.

issue 1) is about encoding guidelines, issue 2) is about editing principles.

Regarding 1) The non-existence of something is usually encoded by not-encoding, i.e. the presence of a co-author is encoded as a second <author>  element, the absence of a co-author can be inferred(!) only by the absence of a second <author> element.
If you knew there was a co-author but you don’t know the name, you’ll need to come up with some project specific guidelines: You could name those authors „Anonymous A“ etc., or you could come up with some reserved string/identifier, e.g. „?“, „unknown“, etc., or you’d introduce a specific (empty) tag, e.g. <author type=„unknown“/> or simply <author/> (where type=„unknown“ might be debatable for the same reasons as <author>unknown</author>).

tldr; In my opinion there is no generalisable way to encode the "not-knowing“ (of such structural features) – we can work around with project specific conventions, where I’d prefer an empty tag (<author>) over a reserved string („unknown“).

Hope that makes sense

> Am 29.10.2018 um 05:31 schrieb Syd Bauman <[log in to unmask]>:
> A question about use of correspDesc/correspAction/placeName arises
> from Julee Tanner's use posted a few hours ago:
>> <correspAction type="sent">
>> <persName>Richelieu</persName>
>> <placeName>unknown</placeName>
>> <date when="1462-06-29"/>
>> </correspAction>
>> <correspAction type="received">
>> <persName>Chavigny</persName>
>> <persName>de Noyers</persName>
>> </correspAction>
> I submit that "unknown" is not the name of a place (although perhaps
> "Unknown" is .. but if you are dumb enough to name your town
> "Unknown" you deserve all the problems you will have getting your
> mail). So I don't like this encoding.[1] But how does one indicate
> that the place it was sent from is not known to the encoders or
> editors? If you just leave <placeName> out, future you won't know if
> present-day you did not know the place, or just forgot to put it in.
> Is there a formal mechanism for saying "this bit not known" in
> <correspDesc>?
> Notes
> -----
> [1] And I am (I think) pretty consistent about this. I recommend
>      <author><persName>Danvers, Kara</persName></author>
>    but
>      <author>unknown</author>