Was it a felony?
If it was a felony, though it has been expunged locally it MAY (and probably WILL) show up in the FBI database that is used for NCIC checks. Every time you are fingerprinted incidental and pursuant to arrest...that info goes to the feds and if there is no disposition of the case, it can come back to haunt you.
IF it has truly been expunged, then it is as if it never happened. You may have to provide some paperwork to the feds and demand an amendment of redords.
"If you take your hands off the back of my cruiser one more time...I will make your birth certificate a worthless document!"