sneak

someone who prowls or sneaks about; usually with unlawful intentions

make off with belongings of others

someone acting as an informer or decoy for the police

marked by quiet and caution and secrecy; taking pains to avoid being observed

“a sneak attack”

pass on stealthily

to go stealthily or furtively

“..stead of sneaking around spying on the neighbor's house”

a person who is regarded as underhanded and furtive and contemptible

put, bring, or take in a secretive or furtive manner

“sneak a look”