Wubei Zhi (Vol. 29)

Forty-fifth (continued): ...Thereafter, if scouts Scouts (Fulu) — soldiers sent to lie in wait or patrol the roads have already departed through the camp gates at dusk and must return to deliver a report, the information must be transmitted through the closed gate. First, one must verify the military signal and distinguish the speaker's voice and accent to confirm they are indeed our own soldiers. Only after this identification is it permitted to step forward and inquire about the details of the report. Once the situation is fully understood, the gate may finally be opened.

Furthermore, when scouts are outside the camp, there is a grave concern that they may be captured by enemy troops. In the darkness of night, the enemy might release a captive to act as a decoy, pretending there is an urgent matter to report to the camp; alternatively, the enemy might employ a ruse to lure our army into a trap. Therefore, one must be extremely diligent in identifying individuals through the gate before allowing entry.

Furthermore, after midnight in a field camp, when the scouts depart, they are initially given a specific password. However, a different password should then be established for use within the camp. This internal password must not be shared with the departing scouts. This precaution is taken for fear that if the scouts are captured, the enemy might steal the signal and use it to infiltrate and cause chaos within the camp.

The practice of "stealing the signal" (touhao) was a common concern in pre-modern warfare, where a captured soldier might be tortured into revealing a password, allowing enemy commandos to bypass sentries.