Don't forget, police departments keep local files for life.
What's on local files? Anytime you ever called the cops for anything, any time your name was mentioned when someone else called the cops and warning tickets (look at the top, It'll be called a "citizen contact report"). Usually, local files are for investigative purposes only...but I would imagine that a police department might share this dossier with other local police agencies and possibly the feds if the Police Chief was being nice that day.
Also, courts have laws on when they may destroy court records. In Wisconsin, felony court records may be destroyed at 50 or 70 years. Misdemeanor crimes may be destroyed at 20 years and some types of court records may be destroyed at 5, 7 and 10 years. May means may, not must.
I would imagine that the cost and liability of maintaining files that are no longer required, means that most courts do an annual shredding or burn fest.