I bet the dog is going inside the house.

Poor fella.
My dogs are so use to going out every 2-3 hours that they would most likely go inside the house if we missed taking them out. It's so easy letting a dog out even if it means taking them on a leash. You have to wonder what else is going on behind closed doors.
I have a had a major neck injury that caused me to become disabled and a part time wheelchair. This hasn't stopped me from taking my girls out. If it did, I'd install a dog door for them and put up a fence if needed.
I don't know if you can prove anything to animal control, but you can always try. Give them a call and report animal cruelty. I've called 911(before my injury) before on a dog living in a closed shed without windows, air flow, ect.... It would bark fiercely when I would run by their house. The owners had a really nice and big home too, there was no reason for the dog being locked in a dark shed. Animal control must of taken action since the dog wasn't there a week or two later.