Client behavior with Zscaler proxy

Hi community,

My customer use ZIA with proxy.pac file now and Authentication Frequency set “Daily” on Authentication settings in admin web. But some user not appears Zscaler login screen everyday and they can see web pages without the login.
What is the criteria/condition to appear Zscaler login screen?

I checked customer’s laptop and see “You are accessing the Internet via Zscaler Cloud” and “You’re logged out of your company’s security service” in http://ip.zscaler.com. Customer’s access is protected by Zscaler?