He has to eat somewhere. If his last call before lunch was on the 'home' side of town, if would only be a few minute drive to get home. He could potentially juggle his calls to try and make that happen, too. And he might not know what his calls were until he went in to work, so couldn't tell her ahead of time.
I do find myself wondering if the text was typical or not. Did she ask him that a lot? Or was it a one-off? That is, what it a routine text, or unusual?
If she was planning to take off, maybe she wanted to be sure she had ample time before he came home and found out she was gone. Just a guess, of course.