Arcee doesn't know why she expected Unicron to be an easier topic, but the weight of what Optimus says hits her all at once and she slumps, looking hopeless. She's close enough to the berth to wrap her hand around the edge of it and she leans against it, heaving air out through her vents.

"I didn't think -- oh, Primus." She looks a little sick; she'd known Megatron had interfered in the worst of ways, but she hadn't ever imagined he'd done so when Optimus was so close. She should have, though. She should have known. "So Unicron's...still out there. Somewhere. Because of Megatron."

She looks angry now, too; Optimus's anger is something she can sympathize with, something she shares to a degree, although it could never be so personal or run so deep for her. But it wasn't that long ago that she had attacked Megatron outside city hall, fueled by rage and spite and bitter upset for what he'd done to Orion, to all of them; if there's one thing she has no trouble understanding, it's that.