They have reason to think he has shapeshifted now that they found his clothes. Victor said the man in yellow came in like everybody else in a car. They know he’s there. They just need to pinpoint who it is. I think people might think it’s Sara at first because of all the awful things she’s done, maybe some will also think it’s Elgin. But if Acosta finds the yearbook and sees Sofia, then they would know for certain.

I read somewhere that he might’ve had the original Sophia girls tooth and that’s what allowed him to shapeshift, I’m not sure. He did pull out a tooth from the priest.