How do i stop my dog from barking when i am not home?

my 10 month old Golden Retriever barks when no ones home and our neighbours are not happy! i have no idea what to do as she never barks when we are home so i can't dicipline her... i have tried a bark collar (one that lets out an ultrasonic noise when they bark) and that has not worked and i would rather not use the ones that zap her when i am not home... we have been told about one that sprays citronella doesn't work either... any ideas would be appreciated - keeping in mind that she does not bark when we are home...