In many MODS records harvested from several sources, I see
<mods:publisher>s.n.</mods:publisher> or something similar (worst I have seen:
000NOPUBLISHER000) to indicate the publisher is unknown or that there is no 'official' publisher. For unknown information, I think leaving elements out makes sense. However, for information known to be missing, something else may work better. For example, for the place of production, RDA (or at least the draft from 2008) §184.108.40.206 specifies
If neither a known nor a probable local place or country, state, province, etc., of production can be determined, record Place of production not identified.
Are there special field values (perhaps "unknown") or structural elements (perhaps an attribute
unknown="true") to indicate 'this value is known to be missing'/'not identified'?
(Edit: added RDA example.)