WordNet (r) 3.0 (2006):

sub rosa
    adv 1: in secret or covertly; "held a meeting sub rosa to avoid
           general criticism"

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "sub rosa":
   behind the curtain, behind the scenes, behind the veil, by stealth,
   covertly, furtively, hugger-mugger, in a corner, in a whisper,
   in camera, in darkness, in hidlings, in secret, in the background,
   in the dark, nobody the wiser, privately, secretly, sotto voce,
   stealthily, surreptitiously, under the breath, under the rose,
   undercover, underground, with bated breath