I'm shocked! I never knew this and asking around, no one else knew it either. That's lettings quite a few children slip through the cracks. If I were a parent of a missing child (knock wood) I'd be very upset if there wasn't an Amber Alert.
However, it turns out that apparently each state sets the guidelines for when it is appropriate to issue an Amber Alert --- so MAYBE some states do require a vehicle description? I know I've read this MANY times.
http://en.wikipedia.org/wiki/AMBER_Alert
Lots of interesting info at wikipedia but it isn't stated that one must be known to have been abducted in a vehicle.
So sorry!!