Thanks so much for your helpful reply.
Usually, when a cat disappears she is hiding because she feels sick.
It is possible the neighbor cat ran her off but where could she go?
Check up in trees if she has claws. Look under anything she could crawl into, bushes, holes, sheds, garages, even discarded furniture, appliances, tires etc.
Walk around shaking a jar of dry food, sit outside with smelly cat food.
You can even bring out the younger kitty in a carrier so Chukkie can smell her.
Put up posters with her picture on trees and posts in a 3 square mile area.
Please let me know when you find her.
Best wishes and prayers for her safe return,