"Is it ethical for an employer to monitor an employee's work habits and job-related activities without his or her knowledge or consent?

Yes it is

Explanation:

It is very important to monitor your employee, his growth and performance and contribution to the system of the company, and more importantly, his habit and is relationship with other in the office, his he a bad egg in the system ? So all this can be done without the consent to avoid the employee faking his behaviour ,this will help you to understand the person and see his flawless easily. So yes it is ethical.