Years ago, police normally used white vans to conduct 'secretive' projects. Some police manuals even explicitly said that white vans should be used for surveillance projects because they are fairly common.
Today, of course, most people are aware that police use white vans that way, so they are not as commonly used for real 'criminal' surveillance. Instead they are often used to harass people and, as the name suggests, to commit rapes and other crimes, and in various other projects.
Anybody can watch videos of the recent protests in the U.S. and count the unusual number of white police rape vans around the protests. The protestors know who are in the vans, and they know the purpose is intimidation, harassment. The police are basically hiding behind legitimate users of service vans, e.g. delivery vehicles etc, the same way a gunman might hide behind children. Occasionally in popular media there are references to this, for example real service van drivers complaining about police 'hiding' behind them.