There is a major overlap between the Mystery category amd Whodunnit.  Arguably, Whodunnits are a sub-set (a specific form of Mystery).  In addition, the Mystery category contains those mysteries that do not involve murder.
There is another sort of play that is not covered by this category, which is the Medieval Mystery Play.  Those were plays on religious subjects, so if we publish anything along those lines, then the Religious and Moral category is the place to search.