The Collaborative International Dictionary of English v.0.48:

Unperceived \Unperceived\
   See perceived.

WordNet (r) 3.0 (2006):

unperceived
    adj 1: not perceived or commented on [syn: unperceived,
           unremarked]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "unperceived":
   able, behind the curtain, behind the scenes, camouflaged,
   concealed, disguised, enigmatic, hidden, imperceptible,
   incalculable, incognizable, indiscernible, insensible, invisible,
   latent, mysterious, out of sight, puzzling, sealed, secret,
   sightless, strange, submerged, unapparent, unapprehended,
   unascertained, unbeheld, unbeholdable, unbeknown, uncharted,
   unclassified, unconsidered, undescried, undetected, undiscernible,
   undisclosed, undiscoverable, undiscovered, undivulged, unexplained,
   unexplored, unexposed, unfamiliar, unfathomed, unheard, unheard-of,
   unheeded, unidentified, uninvestigated, unknowable, unknown,
   unmarked, unminded, unmissed, unnoted, unnoticed, unobserved,
   unperceivable, unplumbed, unrealized, unregarded, unremarked,
   unrevealed, unseeable, unseen, unsolved, unspied, unsuspected,
   unthought-of, untouched, untraced, untracked, unviewed,
   unwitnessed, viewless, virgin