I read the article yesterday. The prime suspect is some sort of gas poisoning, since the dog died too, however it seemed to say the dead dog was closed in a closet. The two other dogs, one inside and one outside, were OK.
If you have a suicide pact, you don't usually take just one dog with you, so I kind of doubt that. It sounds like an accidental leak of some kind that suffocated them. The door being open is likely police, as it mentioned when the person who discovered them and called 911 said the house is fully locked.
My wife was in a car accident yesterday, so I wasn't online much.