Separation anxiety is quite simply a collective term for behaviours from a dog that does not cope well with being left alone. Some dogs will immediately go into a state of panic as soon as the door is closed, whilst others may show low level stress that is visible upon your return, such as chewing damage or barking in your absence.
Extreme panic may involve loss of bowel control, wild attempts to escape the room or self mutilation to name a few. Some owners may be unaware of barking in their absence until it is brought to light by a neighbour for example.
Regardless of how serious the dog's behaviour in your absence, the dog is in a stressed state leaving the task of changing the dog's association towards being left alone for varying lengths of time. Interestingly it has become one of my 'favourite' areas to resolve due to the improvements that I know are possible provided the correct approach is taken. Owner interaction is utterly key with this subject, and we shall be looking at your interaction with the dog as much as the dog's behaviour.
As always my job is to work closely with you until we find a satisfactory outcome for all concerned.