Background: One of the last major Nazi propaganda campaigns was the “Shadow Campaign” of the first half of 1944. It was designed to combat idle chatter and build awareness of enemy spies. This page has most of the posters from the campaign. I’ve also included several images from other sources that were part of the overall campaign.
The source: All of the posters were in color, but I have available mostly black and white copies from Rüstzeug für die Propaganda in der Ortsgruppe #3 (1944).
A Nazi Poster Campaign: 1944

This poster was provided by the son of Technical Sergeant James H. Duncan Sr., an American soldier who took the picture in Leipzig on a poster pillar.
Duncan got to Leipzig in April 1945, which means the poster had been hanging for over a year,
Clicking the thumbnail will bring up a larger image of the poster in color.
It was one of the first posters in the campaign, intended to arouse curiosity about what it meant.

One man tells another: “It’s over there at the edge of the forest...”
“Pst! The enemy is listening.”

A barber says: “I can tell you that on...”
“The enemy is listening.”
Image courtesy of Dr. Robert D. Brooks.

The theme shows up in this cartoon from the 28 April 1944 issue of Lustige Bllätter, a satirical weekly magazine.
The soldier on leave says “The new guns have a calibre of...” Before he can finish she responds: “Pst, Heinz! No one may hear that!”

Newspapers carried items supporting the campaign. The translation of this example:
“Pst! Didn’t you say a little too much yesterday?
Don’t do it any more, not any more! Be quiet!
Pst! Watch yourself! And warn your friends! Today any careless word is poison!”

The theme even showed up on matchboxes.