The Collaborative International Dictionary of English v.0.48:

Unnoticed \Unnoticed\
   See noticed.

WordNet (r) 3.0 (2006):

unnoticed
    adj 1: not noticed; "hoped his departure had passed unnoticed"
           [ant: noticed]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "unnoticed":
   behind the curtain, behind the scenes, camouflaged, concealed,
   disguised, hidden, ignotus, imperceptible, indiscernible,
   inglorious, insensible, invisible, latent, little known, nameless,
   no credit to, obscure, out of sight, overlooked, renownless,
   secret, sightless, submerged, unapparent, unbeheld, unbeholdable,
   uncelebrated, unconsidered, undescried, undiscernible,
   undiscovered, undistinguished, unfamed, unglorified, unheard,
   unheard-of, unheeded, unhonored, unknown, unmarked, unminded,
   unmissed, unnotable, unnoted, unobserved, unperceivable,
   unperceived, unpopular, unrealized, unregarded, unremarked,
   unrenowned, unseeable, unseen, unsung, unthought-of, unviewed,
   unwitnessed, viewless