I need an explanation for a situation that just happened?

My husband and I were keeping a dog for a couple we know. My husband picked him up at the owners house, claims to have never gone out the back door and did set the alarm when he left. The owners returned and said when they got home the back door was opened and the alarm was not set and that they know they locked the back door when they left. My husband is a responsible and honest guy and has picked up and dropped off the same dog a few times before without incident. So, what explains this situation besides that he walked in, left the back door open and failed to set the alarm. I know he would not do it intentionally and struggle to think he would do it accidentally as he is uber responsible. If he doubted any of it I think he would tell me "I am not certain" instead of 'I know for sure." These are really nice people and we do a good job for them. I wouldn't want to let them down. Is there anything else that could have possibly happened? If so, what?