Don't forget that potential thief may have a team (at least two people). I have seen many booths have at least two workers. It depends of the size of the booth and how much vulnerable your booth is.
The thieves will observe the security, surveillance system, escape routes and the booth layouts first before they execute the plans. Of course, the thieves will prepare themselves if one of them is caught. The book must be abandoned immediately. Other scenario, they may need each other as the relay team like the stolen book will pass through other thieves without being noticed or caught.
I have a feeling for the dealer who was recently distracted by one of the thieves while his book was snatched without being noticed. The walls are the most vulnerable even it can be fell off easily. It has nothing to do with your eyes even in ages. It is simply too many people flocking in a long day even the security system is not always helpful. It can happen so fast before your eyes will blink.