A couple thoughts--
-We don't know the whole convo about the book fair and the money. Maybe she thought $25 was a lot for three books and that prices were a bit marked up at the thing and called him saying something like "I bought only three books there and they charged me 25 bucks for just that! Can you believe how pricey it is?" to which he replied, "That's ok." We just don't know the tone or exactly what she said. Additionally, he may not have said exactly "that's ok" or he may have added to it in the real convo--he was just quickly recapping in an interview.
-IMO it's important to know if the door was open when he got home or not. Closed door? I can see it not being that unusual that he would think she just ran to a friend's home in the complex if that's what she did a lot. Honestly, if I was just walking to a friend in the same apt. complex, I probably wouldn't bring my purse, wallet, etc. I would lock my apartment door, though. So if it was wide open, that changes things for me.
-Though if it was wide open, I wonder if it sometimes didn't latch all the way? I'm thinking of a scenario (besides the aforementioned laundry room) where she'd take baby out of the apartment and leave everything but the phone behind, thinking she'd return soon. Getting the mail is what comes to my mind. Most apartment complexes have all the boxes in one communal area rather than a box outside of your own apartment. So she sees the mail truck or just always gets mail at the same time or whatever, leaves everything behind but has her phone in her pocket, either purposely leaves door wide open (afraid of getting locked out?) because the mailbox is super close OR kicks it shut behind her but it doesn't latch all the way and stays open and then someone intercepts her while getting the mail. Just a theory, obviously MOO, but I'm thinking of other scenarios where the scene would be left like this. It's near the holidays, she may have been expecting a package, etc.