Speed Cameras - Covert or Obvious?

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I recently read a commercial transport magazine article about police speed cameras and how it has been fairly well publicised that they should be located (permanently or mobile) in places which can be seen easily by the approaching driver. Further, I understand that good warning signs are supposed to be in place well before the location of these cameras. According to my understanding, the article reported that the police were caught using speed detection cameras from the back of a horse box, yes ... a horse box - which was parked in a prime location for their easy pickings. Do you think that this sort of activity is right and proper behaviour ?