When something is expunged, how can an employer find it!?

I applied to be a youth care worker. I had an interview and the people hired me on the spot for not only the summer youth care position, but also for the residential worker. When asked was there anything that would show up on my background, because my record was expunged, I told them "NO" confidently. I started my first day of work and everything. The next week I got a phone call saying that before I return to work, I would have to talk to HR because things came back on my background check. When I talk to the lady that hired me I told her that, in repsect to my rights, I did not have to tell them about my record that was expunged. She then recommended that I speak with HR. What should I do and how could things that were expunged possibly come back on my record when I went to court to get them expunged and I recieved letters saying that my record was expunged? What could have possibly happened and what should I do? Do I make a fuss because the stuff was expunged? Should I sue them and would I even have a case against them? What should I do?