Wasted opportunity?
In the first conversation with Mrs. Asher, she talks about how her son had a nervous tick where he'd pull on his ear and she couldn't get him to break that habit.
From the beginning I was sure that Ethan Hawke's character was going to end up being the killer, but I thought they'd do it by having him slip up around Illeana and pull on his ear in that way which would tip her off. Since she was presented as being hyper-observant and they made it a point to explain that identifying characteristic of Martin Asher, it seemed natural that she would find out that way.
So they introduced her attention to detail, his nervous tick, and made the two of them get closer and closer throughout the investigation... only to have Illeana discover the truth when he's covered in his mother's blood as she's lying dead on the floor of the elevator.
It was just kind of a let down that Illeana found out because of something so blatant instead of a more subtle reveal based on previously introduced aspects.