Cloak-and-dagger

It's a cloak-and-dagger operation.

There's some cloak-and-dagger stuff going on here.

Israel wages cloak-and-dagger war on Iran [headline]

Two years after ordering the mass internment of German refugees at the start of the second world war, the British government put a small, elite group of them in military uniforms and sent them on a secret cloak-and-dagger mission to occupied Dieppe to snatch an Enigma coding machine from under the Nazis’ noses.