WordNet sense 1 (marked by deception):

underhand, underhanded

WordNet sense 2 (not straight; dishonest or immoral or evasive):

corrupt, crooked, crooked

WordNet sense 3 (marked by quiet and caution and secrecy; taking pains to avoid being observed):

furtive, stealthy, surreptitious, sneak

WordNet sense 4 (hidden on any grounds for any motive):

concealed