Assalaamu alaykum. There is a traffic company that is offering a job that is a part of an improvement project. They are offering extra jobs as a "mystery passenger" where your job is to check up on the quality of the staff and the enviroment of the company by asking questions to the staff and do observations. What I have understood is that your are working anonymously or pretend that you are a regular customer that has questions. For example, you ask questions to see how the staff react and you test their knowledge about a specigic thing, but you do not tell them that your are doing an observation or that you are working anonymously. However, I do not think that you mention any names, just that you report in general about the things that you asked. Then you report the results to the company. Now, is this job halal or not? Is it like you are spying on people? Or does it constitute backbiting? Is this job halal or haram? May Allaah bless you. Wassalaam.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

In principle, there is no harm in working in that job because assessing employees, verifying the quality of the work, and writing reports to the concerned authority are all lawful. These are among the mechanisms of work organizations and for improvement in order to address any potential shortcomings and defects.

This is not considered spying as some may mistakenly assume. Spying means searching for the faults of others and seeking out their shortcomings. Ibn ʻUthaymeen wrote, “Spying means looking for the faults of others whether that is done directly, i.e. that the person goes himself to spy perhaps he would find a fault in his brother, or by using devices such as a recording device or over the phone. All that helps the person find the faults and shortcomings of his brother comes under the heading of spying, which is prohibited in Islam...” [Sharh Riyaadh As-Saaliheen]

This is not the case with this job; you are not looking for the faults of the employees. Rather, the job entails monitoring the performance of the employees and assessing their work, and sometimes this requires secrecy so as to serve the best interests of the work.

Allaah knows best.