sneaky

Etymology

From sneak + -y.

adj

1. Difficult to catch due to constantly outwitting the adversaries
   Catching those thieves will be hard: they're so sneaky!
2. Dishonest; deceitful.
   They played a sneaky trick on us.

noun

1. (espionage, slang) Any device used for covert surveillance.
   […] in cooperation with the National Security Agency, installs and maintains "sneakies" throughout the U.S.S.R. and Communist China — but increasingly, denied areas are surveyed more simply. 1974, Miles Copeland, Without cloak or dagger: the truth about the new espionage, page 244
   […] has used travellers to plant 'sneakies' - small electronic transmitting devices which form part of a surveillance network. 1991, Chapman Pincher, The Truth about Dirty Tricks