Criminal Law Questions? Ask a Criminal Lawyer.
For the most part, there is no legal requirement of any law enforcement agency to let you know if you are being investigated or if you were investigated in the past -- however, if you can tell me some more specifics here then I might be able to give you a more specific answer.
As I said, if this is a law enforcement investigation they do not have to notify you of anything -- period. You may get some idea that an investigation is happening if law enforcement personnel are asking a lot of questions to many people that you know (family, friends, neighbors, co-workers) and if you get wind of it you can certainly call the agency and see if they will tell you what the matter is about (they may or may not depending upon what the situation is all about). ABout the only time you have a right to know or learn anything is if the law enforcement personnel contact you directly and are looking for a statement. Even the child services department can quietly investigate you without your knowledge and then confront you with the results of the investigation as they are taking your kids away from you. I really wish I could tell you otherwise but for the most part, law enforcement agencies conduct investigations in complete black out conditions until they want you to know you are being investigated.
As I started reading your response when you had written "innocent until proven guilty" it popped right into my head -- "but only in a court" -- and you answered your own question in the next line. I think you realize that wiretapping and gps tracking all happen and are used by law enforcement agencies and the person never even knows they are the target of an investigation (that would defeat the purpose of these things like wiretapping if law enforcement wants the suspect to say something incriminating -- if he knows he is being investigated then he will not say anything). The way that these things are challenged is in a court during the trial -- the defense will put up a Motion to Suppress evidence and then try to make out a case to the judge that these things should not have been done and if that is the case then the matter will go no further and there will not be a trial. I am not saying that I agree with all of this, mind you -- and the manner that the police get warrants for wiretapping or warrants to search and seize is that a law enforcement officer swears out a warrant to a judge and declares that they have enough "probable cause" to put in a wiretap or to search the home -- and cops either lie ("testilying" we call it in the legal field), and they base many warrants on "confidential informants" (which, as far as I am concerned, throws the right to "face your accuser" right out the window in our legal system which is WRONG -- but the courts have consistently held that the CI's safety is more important than the constitution). SO, I agree with you here and many things that happen disgust me also --- but the cops or other law enforcement do not have to inform you or anyone if they are investigating a particular person.