neighbors dog

Our neighbors adopted a dog from friends that didn't want him anymore,not real sure of the breed. He's medium size with brownish hair and yellow eyes,about a year old. After they had him a few months he started to go to the bathroom in the house on occasion,even when they were there. He'll go upstairs when they are downstairs. So he knows he's doing wrong,but why? He gets walked every day and is rarely alone. Can you give us any ideas why he would do this?