Need opinion - Indoor Drain Clear Out

Had a sewage backup. Real fun weekend. Before restoration company can begin repair, they needed us to get a plumber to locate the Indoor Drain Clear Out. Northern Climate, no way it's outdoors. Plumber showed up and searched for an hour and a half. Couldn't find it. We believe it's the drain in attached photo, he disagreed and wouldn't even run his camera down there for risk of "damaging his $23,000 camera." Any thoughts? We spent 5 hours tearing apart the basement and there are no other candidates.