If your messing around my house when my owners are gone then mainly to scare you off. I will protect my territory.

They are attempting to bark loud enough that the owners will hear them, even if they are in another state. I suspect they are saying things like: "You are leaving me!? How dare you leave me!" /pads over to watch you pull out of the driveway "If you aren't home soon I swear I will make you sorry!" /begins to look for any bits of food you might have left on the counter "Okay, be advised! I am now going to knock over the trash!" /knocks down the trash shreds random items "Still think you can ignore me like this!" /feeling the need for some deny ability the cat chasing begins "HA HA HA Look at that cat run!" /eats some garbage to make you feel both angry and guilty "You need to be home now! How dare you not take me and leave me with this damned cat!" /checks the driveway again "You are taking way too long! You better bring me steak!" /chases cat then stops to consider what item he can pee on or chew "Okay! It's war! I am going to bark until the neighbors hate you!" /checks driveway to see you arriving home "OMG OMG OMG You came home! OMG OMG OMG" /checks you for steak but when none is found walks away giving you the cold shoulder for leaving happily knowing your will be punished by your neighbors for leaving him home with the cat

WHAT THE HELL I GOT A BIG SHIT BREWING HERE!

Barking loudly or at all can be a mix of reasons, from a Dog's point of view! Most dogs are rather territorial. They take protecting their home pretty seriously. When they are alone and have no "back up" so to speak from their "pack" (the family) they want to be sure that anyone outside or on their property is aware of their presence and warn them off. Then too, many dogs are slightly (or in some cases extremely) stressed when left home. Stress can cause a dog to do behaviors they might not always do, or might not to to such an extreme! The one thing you can count on, is that if your dog does an unacceptable behavior when left alone, they are NOT trying to "pay you back" by being naughty. Their brains can not rationalize in such ways. The last thing a dog WANTS TO DO...is piss off the humans in their life. Most people, when annoyed or downright angry, act or vocalize their ire in some fashion. These sorts of angry or upset actions or vocalizations of OURS usually SCARE our dogs...They don't like us upset for any reason...not at each other, and sure as heck...not at THEM! Some dogs even react to a beloved or other person crying...it doesn't have to even be related to the dog, it could be anything that upsets us...Some dogs may seem to try and offer comfort or run and hide when their human is sad or upset... The last thing a dog would think to do is piss you off on purpose...not gonna happen!

Some do it out of separation anxiety. I know mine does, but not when he is left in his little house (his crate.)

They are doing it out of an urge to communicate. They get the point across to you pretty quick that if you intend to do anything to their house it's not going to go undefended.

It's probably separation anxiety.

they miss them