After making his research, he also felt like everything was too clean and intentionally refused Chris's offer to help him look into it.
After they met again, Chris asked if he trusted in the footage Ashley gave him?
That's when Ethan thinks twice that all the evidence that makes everything appear to be smooth, comes from Ashley.
He is someone that builds relationships on trust and doesn't doubt people easily. He told Chris that he will get to the bottom of it and he did.
Ashley played smart, but Ethan only needed to suspect her before catching her. Because, as long as he didn't suspect someone, he won't find anything on them, since he won't be looking.
He started paying closer attention to her. He could see how his kids were being showered with love and praises, clearly showing that she's trying to get closer to them.
But being the stubborn kids that they were, the two never responded to her.