Elijah had been happy to receive the invitation for a quiet evening with Rebekah. That they'd reached a place again where they could just be has been a blessing, and one he's loathe to let go of.

He's startled by the knock, as well, thinking it was just going to be them--and then concerned, immediately, when he hears his mother's voice. He follows her toward the door, stepping into his mother's vision as she asks the question.

"I'm here. What's wrong?"

Because clearly something is, and his mother being upset is never a good sign of anything for anyone, he's decided.