Is not returning a greeting by some managers who pass in front of employees, and then mentioning this characteristic in front of colleagues, considered backbiting that necessitates expiation?

Returning greetings is a commendable Sunnah, and not doing so does not necessitate backbiting, especially since it would be considered backbiting against a specific person as long as some of those present know whom you mean.

You are obligated to repent to Allah Almighty by feeling remorse and resolving not to return to this sin. You are not required to inform that manager that you backbit him, if you know or strongly suspect that informing him of your backbiting him could lead to mischief.