The answer to this question is very simple to figure out but people still confuse what exactly a radar detector is designed to do. You see, this device is not designed to detect police cars or vehicles. Instead, it is engineered with the sole purpose of detecting certain frequencies it recognizes. Their main purpose is to detect radar guns but that is not the only frequency that they are going to pick up (which goes back to the importance of sensitivity).
Not all policemen are going to scan your speed when you pass them or when they pass you. So, if your radar does not go off, it merely means the police officer did not try to scan you (unless they were using a band that your device does not pick up).