According to the various sources there appear to have been a number of plays by this name.

Het Geheim by Christian August Vulpius (1762 – 1827)

A German play in 5 acts by Christian August Vulpius (1762 – 1827).

Performances in South Africa

1855: Performed on Tuesday 12 June 1855 by Door Yver Bloeit de Kunst in the Garrison Theatre, and credited to "the author of Rinaldini" - i.e. Vulpius). Also performed were De Helleveeg (Loosjes) and Het Groot Ballet. The evening was in support of the "Patriotic Fund" for the Crimean War. The production was possibly repeated on 28 June.

Sources

F.C.L. Bosman, 1928: p462-3

Het Geheim by François Benoît Hoffman

A French comedy by François Benoît Hoffman (1760–1828)

Translated into Dutch by Pieter Gerardus Witsen Geysbeek. Published by Abraham Mars (Amsterdam), 1801.

Sources

http://books.google.co.za/books/about/Het_geheim.html?id=holWAAAAcAAJ&redir_esc=y

Het Geheim by A de Bruine

The last production by Door Yver Bloeit de Kunst under the leadership of the 71 year old Johan Combrink, at the Good Hope Theatre 4 June, 1910. Boonzaier wrote a devastating criticism of what was apparently a ludicrous melodrama, badly presented, thus it was most probably a Dutch version of the Vulpius text.