Ah ha! This article finally explains what happened to me last year at the SFO airport port-of-entry. I always declare any food items at customs and on this trip we brought back a good sized chocolate Easter egg that the Hilton Rome Cavalieri gave us.
We got tagged for a secondary and the inspector zeroed in on the chocolate egg. He said he had to run it through the x-ray to make sure it was empty. Which it was. He then gave us back the egg and I asked him what he was looking for (I'm thinking drugs). He explained they look for small toys that can be a choking hazard for children. I thought that was a little strange at the time.