Ella did know what he wanted her to do, but she reached for the nearest shirt, anyway, which ended up being the one he'd worn the day before. It swallowed her whole as she put it on, but she rolled up the sleeves as she watched Terrence move. She didn't assume he wanted her to follow wherever he was going, and she wasn't one to do so in the first place. But she at least needed to be wearing something when she went back to her flat, if he chose to leave.
Did she want to be where he was? Yes. But if he wanted to be somewhere by himself, her to leave him to his own devices, she would. It wouldn't hurt her feelings because that wasn't how things worked between them. So she waited, kneeling on his bed, while he did whatever it was he was going to do.