The FBI may be the only ones who may still have access to those records. What happens is when a case is expunged it will be erased from the court's systems, police departments, or any departments that you were involved with. No one will be able to look it up. The court's may be able to but cannot disclose any information.
Future employers can find out so can the people who grant you a visa
Only the court can look in sealed records no can see expunged records unless you apply for a job in law enforcement