In Chapter 12, of "The Hiding Place," to what does the "thick clouds" refer?

In chapter 12, of "The Hiding Place," Corrie is being moved from one camp tho another. When she and Bessie arrive at Vught they are given separate work assignments.  Corrie is put to work in the Phillips building soddering wires on to components for airplane radios.  When the guards leave the building the prisoners stop their work and rest, visit, and communicate with each other.

When one of the workers, sitting by a window sees a Nazi officer returning, they yell: "thick clouds, thick clouds."  The prisoners would immediately return to work.  When the officer walked in the room to check on the production for the day they don't have a clue that the prisoners were on a break.

The term "thick clouds, thick clouds," was a code to the prisoners that one of the guards was on  the way back to their building.