I feel it

She's been gone for a few days and my other cat looks so sad all the time now

I hope so. Thanks, meep. :)

@dimebag
Set out a pillow case or something that smells familiar to them, favorite toy. Also go outside and just chat with someone. They'll respond better to your calm voice. More tips here.
https://petfbi.org/i-lost-a-cat/tips-to-lure-a-cat-back-home-2/