Obviously, this contains spoilers through the S4 episode "Tuesday" ---
**
Carrie is willing to consider three alternatives:
1) Marshall works for the most evil sonofabitch on planet earth;
2) Marshall is having an affair; or
3) It's perfectly normal for husbands not to come home for 24 or even 48 hours at a stretch, and she's just been out of the workplace so long she's forgotten.
The alternative she won't consider is that he's still working in intelligence. If that's true, Marshall will always be like this. Forever rushed and tense. Forever in danger.
Carrie holds Mitchell closer, looks out at the still-empty driveway, and hopes Marshall's having an affair.