As Mr. Brooks pointed out, 1203.4 dismissals (ie. expungements) apply to convictions, not arrests. And also as he pointed out, if no case was filed and the statute of limitations has run (meaning it is now too late for a prosecutor to bring charges), then the arrest is likely to actually be deemed as a detention. Did you do a background check on yourself to see if anything comes up? Also, how long has it been since the incident occurred?

I don't know what state you are in but in California, expungement applies to convictions; not arrests. In CA it is very difficult to get an arrest removed from the DOJ database. However, under California law, when no case gets filed, the arrest is legally deemed to have been only a detention.