Learn Polish: Według mnie szef podsłuchiwał pracowników bez powodu. - In my opinion, the boss was eavesdropping on the employees for no reason.

In this context, the prefix pod- adds a sense of stealth or secrecy to the action. By attaching it to a form of słuchać (to listen), we get podsłuchiwać, which means "to eavesdrop" or "to listen in secretly."

Since prefixes usually make a verb perfective, why is podsłuchiwał translated as an ongoing action ("was eavesdropping")?

You are right that adding pod- to słuchać creates the perfective verb podsłuchać (to eavesdrop once). However, the verb in this sentence has been made imperfective again by adding the suffix -iwać. Therefore, podsłuchiwał is the imperfective form, showing that the boss was in the process of eavesdropping over a period of time.

The base verb słuchać requires the Genitive case. Is pracowników in the Genitive case here?

Surprisingly, no. While słuchać takes the Genitive, the prefixed verb podsłuchiwać takes the Accusative case. For example, you would say podsłuchiwać rozmowę (Accusative). Because pracownik (employee) is a masculine personal noun, its Accusative plural form (pracowników) just happens to look exactly like the Genitive plural.

Why do we use the pronoun form mnie in the phrase według mnie?