Finding an Invisible Target Too Quickly: Analysis of a Sequential Search Task Anomaly in 36 Million Trials

In an online sequential card-search task, the target card position was generated server-side and stored in a location inaccessible to the participant. Back-of-card images presented to the participant were identical across all five positions, and no target information appeared in any client-accessible form. A line-by-line code audit confirmed no pathway through which target information could reach the participant. Under these conditions, 106,996 participants contributed 35,945,094 trials from 2002–2018. Overall, they found the target in slightly fewer steps than expected by chance (mean = 2.9975 vs. 3.0; z = −10.73, Cohen's d = 0.0018). Although the deviation was extremely small, its direction and magnitude were estimated with high precision.Permutation tests preserved both the observed distribution of user click patterns and the exact distribution of generated targets while disrupting only the trial-level pairings between them. The observed mean fell more than 10 standard deviations below the permutation null mean, ruling out explanations based on interactions between user behavior and target distributions. Additional analyses found no evidence that target autocorrelation, duplicate records, pseudorandom algorithm artifacts, or behavioral strategies accounted for the effect.A preregistered analysis of data collected from 2021 – 2026 on an independently developed web platform found a consistent below-chance effect across approximately one million trials. However, data irregularities found in this study’s dataset precluded treating it as a satisfactory replication; it is best interpreted as convergent evidence, with primary evidential weight resting on the first study.Across both datasets, a small but precisely estimated deviation from chance was observed under conditions designed to preclude access to target information. The deviation survives permutation testing, code audit, and replication on an independent platform. Its origin remains unexplained.