Mom was often seen feeding the chick and after a few days there was no more sign of it. No body or other evidence of foul (fowl!) play. Looks like the baby got its wings and flew off with mother. As an aside, the morning after I first discovered the foundling I also found two other chicks just outside the dog pen wire. These guys were evidently more advanced than their sibling, because when I approached them they flew off into a nearby cottonwood. It wasn't an elegant flight, but it did the job.
Emma still circles the fence looking for the chick, but she is about to give that up in favor of more profitable patrols.