It had been such a long day, prolonged by unsavoury thoughts and lack of sleep, and all Ares wanted to do was have a shower and pass out on the couch. It's not like Phobos could die and stay dead but he knew how ephemeral things could be. Eros and Psyche had came and gone, and all he had left were Phobos and Deimos.
Getting into yet another argument with Aphrodite didn't help, and while he wasn't angry enough to just pick up the pieces of his family and disappear off somewhere, he was adamant about not letting Zeus anywhere near his kids. They would either do this Ares' way or nothing would be done at all.
He had told Deimos to stay here in case Phobos returned, so when he opened the front door, he thought it was Deimos making the racket.
Except he heard the voice, and it wasn't Deimos', and the sound of clinking keys dropping on the ground was deafening.
He wasn't sure when his feet carried him to just outside the kitchen where the voice originated from, but they did, and sure enough - Phobos. Just standing there, as if he'd been there this whole time.