The Collaborative International Dictionary of English v.0.48:

Undiscoverable \Undiscoverable\
   See discoverable.

WordNet (r) 3.0 (2006):

undiscoverable
    adj 1: not able to be ascertained; resisting discovery [syn:
           unascertainable, undiscoverable]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "undiscoverable":
   able, beyond reach, closed forever to, closed to, enigmatic,
   impenetrable, impervious, inaccessible, incalculable, incognizable,
   lost to, mysterious, out of reach, puzzling, sealed, strange,
   un-come-at-able, unaccessible, unapparent, unapprehended,
   unapproachable, unascertained, unattainable, unavailable,
   unbeknown, uncharted, unclassified, undisclosed, undiscovered,
   undivulged, unexplained, unexplored, unexposed, unfamiliar,
   unfathomed, ungettable, unheard, unheard-of, unidentified,
   uninvestigated, unknowable, unknown, unobtainable, unperceived,
   unplumbed, unprocurable, unreachable, unrevealed, unsecurable,
   unsuspected, untouched, virgin