What is publication bias in a meta-analysis?

Researchers undertook a meta-analysis of the effectiveness of non-benzodiazepine hypnotics, including eszopiclone, zaleplon, and zolpidem. Randomised controlled trials were included if they were double blind, placebo controlled, and had parallel treatment groups. Participants were adults with primary insomnia (transient or chronic). The main outcomes were polysomnographic and subjective measurements of sleep latency.1 Thirteen trials were eligible for inclusion. When the results of the trials were combined, non-benzodiazepine hypnotics showed a significant improvement (reduction) in the main outcome of polysomnographic sleep latency compared with placebo (weighted mean raw difference −22.0 minutes, 95% confidence interval −33.0 to −11.0). Non-benzodiazepine hypnotics showed a small improvement in subjective sleep latency although the change was not significant (−6.9 minutes, −26.0 to 12.4). Improvements in sleep latency did not vary between individual hypnotics. The researchers also reported that Egger’s test showed no evidence that the results were significantly affected by publication bias. Which of the following, if any, would have resulted in publication bias?