The birth certificate. Come on, a grown woman could have not gotten along without a birth certificate. Even without the paper document the official records would have been obvious Sherrie wasn't her dad. Lazy writing.
Towards the end after Micah talks with Ivy and leaves the police station, her and Poppy are outside sitting on the bench talking and you see a female police officer and a male wearing a black shirt and blue jeans walk past them in the background. A few moments later, you see the same couple walking behind them again in the same direction.