Behaviour detection officers' task is to spot potential criminals in public spaces, but scientific research concerning what to look for is scarce. In two experiments, 52 (Experiment 1A) and 60 (Experiment 2A) participants carried out a mission involving a ferry crossing. Half were asked to smuggle an object; the other half were non-smugglers. In Experiment 2A, two confederates appeared to approach as if looking for someone on the ferry. Smugglers, more than non-smugglers, reported afterwards to have felt nervous, self-conscious, and conspicuous and to attempt behavioural control during the ferry crossing. The secretly videotaped ferry crossings were shown to 104 (Experiment 1B) and 120 (Experiment 2B) observers, tasked to identify the smugglers. Although they reported paying attention mostly to signs of nervousness, lie detection accuracy rate was poor (48% in Experiment 1 and 39.2% in Experiment 2) because their perceptions of nervousness did not match the experiences of nervousness reported by the (non)smugglers.