I took it as Takeda (family name/surname) Hitoshi (given name) so he would introduce himself as Inspector Takeda Hitoshi and then the others would call him by his title and surname or just his title so Inspector Takeda or just Inspector since we're all strangers. Generally only close friends and family members call each other by their given names. Letting someone call you by your first name is kind of like letting them know that you consider them to be a close friend. Another example is Alexandra Graveson who most of us would address as Miss Graveson, Graveson Sensei or just Sensei/Teacher.