I don't know if I'd call it an "investigation" as much as a simple, honest question. No? If you are asking here on a forum, why not ask someone at the agency instead, like maybe the person who is responsible for paying people and who sees other photographers invoices.
Anyway feppe, it's valid research, IMO. I don't go around giving anyone a percentage of things unless I know it's legit. He's already lost the job ("burned the bridge") by saying no to the fee, so why not ask instead next time?
What if the agent had said 30%? Would you ask if it's normal business practice then?
I have my doubts about just how "influential" this AD is going to be in the long run if he goes around car jacking people in the industry. He won't last long. At least I don't think he would in NY. He'd be fired a few times, and then he'd have a hard time finding work. People talk.
But again, if this is common at their advertising agency (they all get 10% when they connect photographer with client), then that's okay if you want to play spit ball with them. Like I said, charge an extra 10% Spit Ball fee next time you work for them, and you'll be Even Steven.
I don't think I have ever lost a job by asking legitimate questions about billing.