Cat question

One of my barn cats has left the scene. She has been gone for about a week now. And, I am not sure what would prompt this.

Phoenix is a little female - she has been spayed and has all of her shots. She is about 1 yr old. JohnDewey is a little male - he too has been fixed, shots, etc. He is about 6mos old. Both cats always got along, I fed them separately, and they stayed in the breezeway at night.

During the day they both were always around.

Any input?

Thanks,
Melissa