The Collaborative International Dictionary of English v.0.48:

Stealthiness \Stealth"i*ness\, n.
   The state, quality, or character of being stealthy; stealth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stealthiness
    n 1: a disposition to be sly and stealthy and to do things
         surreptitiously [syn: furtiveness, sneakiness,
         stealthiness]