When Did Arbogast Become Suspicious?
At what point did Arbogast start to suspect that Marion had been at the Bates Motel??
1) The obvious choice would be the first time he caught Norman in a lie. Norman had said no one had been there in weeks, but then admits a couple had stopped for directions. While this could have been an honest mistake by Norman, or maybe he meant no one had stayed there in weeks, it still set off some alarms in the PI's mind.
2) When Norman tried to refuse to look at the picture.
3) Or perhaps he was suspicious even before he met Norman. Just pulling up to the ramshackle Bates Motel with no lights on and as he described it "the only motel that seems to be hiding from the world." would be enough to make him think it would be the perfect place for someone who wants to hide from the world. Notice he didn't spend much time talking to the other hotel owners. They looked at the picture, said no they hadn't seen her, and he left. But he didn't do that with Norman. Even after Norman said no one had been there in weeks, he stayed and continued asking questions, even though there was no indication that Norman was lying.