sneaky

Elusive; difficult to capture or observe due to constantly outwitting the adversaries.

Catching those thieves will be hard: they're so sneaky!

Be sneaky. Fish frighten easily.

Dishonest; deceitful.

They played a sneaky trick on us.

A contribution to a wall newspaper of which I was an editor in our sector poked fun at the never-changing menu in the school cafeteria. It centred on the various and devious methods used in serving us hamburger. One time it would be called hamburger, another time bifshtek, then cutlet, and sneakiest of all, schnitzel—the only difference among them was usually only in their shape or whether bread crumbs had been used.

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.

[…] has used travellers to plant 'sneakies' - small electronic transmitting devices which form part of a surveillance network.