Any device used for covert surveillance.
"The detective installed a tiny, sneaky camera in the smoke detector to catch the suspects in the act."
marked by deception
"achieved success in business only by underhand methods"
marked by quiet and caution and secrecy; taking pains to avoid being observed
"a furtive manner"
"a sneak attack"
"stealthy footsteps"
"a surreptitious glance at his watch"
Difficult to catch due to constantly outwitting the adversaries
"The cat is a sneaky hunter who always slips away just as his prey gets close enough to strike."