According to [this review][1] of "Secret Santa" episode,

> I’ll say up front that SyFy very specifically-stated (in multiple places) that "Secret Santa" would not address anything related to Myka leaving on the September finale.

Also, from http://www.tv.com/shows/warehouse-13/secret-santa-1365486/ - not sure whether the info is canonical or someone pulled out of thin air:

> This episode is set out of continuity, as it takes place prior to the events of the preceding episode, the season finale, when Myka resigned as an agent. 



  [1]: http://www.tvfanatic.com/2010/12/warehouse-13-review-secret-santa/