The key not being in the mail box is such an important point, I think.
Bob's long-term housekeeper said he was a man of habit and that he was nearly always there when she was due to call. But if ever he went out, he always left the key in the mailbox. That's why she was so worried when he didn't answer the door but the key wasn't there either.
It may not be connected of course, but I do find it interesting that when son in law returned to Bob's home he seemed surprised to find the housekeeper there - as though Bob hadn't told him she was coming.
The story posted earlier about the husband who murdered his wife but overlooked the tiny detail about leaving the burglar alarm on (would anyone abducting his wife really have bothered to reset the alarm?) sets my mind racing in relation to Bob's case, and those tiny details.