There are some civilian crime investigation shows or movies around at the moment, such as Lie to Me, Castle, Psych and even Sherlock Holmes. Sometimes in these stories it comes up as an adventage not to have to follow police procedure, which I think is a dangerous situation to get into, especially for the police who work with these talented civilians. Technically, I think their police minders should be arresting them for, say, breaking into a house without a warrant, mistreating a witness or otherwise sidestepping police rules. The usual process, however, seems to be to look the other way when this behaviour helps an investigation.
Mokalus of Borg
PS - I can imagine the court cases getting quite a bit more difficult as a result.
PPS - Especially if evidence has to be thrown out.