The SOE manual on counter-surveillance

The Special Operations Executive manual lays out routine counter-surveillance for agents in occupied Europe. An agent should watch for strangers near a departure point, avoid a direct route, use public or private transport without revealing the destination, and enter at least one crowded place. If surveillance is suspected, the mission and any contact with another agent should be postponed without showing alarm. The manual also warns that strained nerves can create false suspicions, so the agent must test whether a watcher is real and remember that several watchers may be working together. Crossing a road, stopping around a corner, entering an empty shop, jumping onto a bus, or abruptly changing direction can expose someone who follows.