WordNet sense 1 (someone who prowls or sneaks about; usually with unlawful intentions):

prowler, stalker

WordNet sense 2 (someone acting as an informer or decoy for the police):

canary, fink, sneaker, snitch, snitcher, stoolie, stoolpigeon, stool pigeon

WordNet sense 3 (make off with belongings of others):

abstract, cabbage, filch, hook, lift, pilfer, pinch, purloin, swipe, nobble, snarf, abstract

WordNet sense 4 (pass on stealthily):

slip

WordNet sense 5 (marked by quiet and caution and secrecy; taking pains to avoid being observed):

furtive, sneaky, stealthy, surreptitious

WordNet sense 6 (hidden on any grounds for any motive):

concealed

WordNet sense 7 (to go stealthily or furtively):

creep, mouse, pussyfoot