Oh dear, sounds like something has spooked her, it's dark so early now it could of been shadows at the window, fireworks or car headlights, did you leave the lights on? Also if she has the run of the house it may be shadows and the like in other rooms causing her to panic and get spooked. The other option is did you feed her before you left her? If so the barking could well of been due to distress at needing the toilet and not knowing where to go.
Also did someone come to the door, or bang on your windows? Where we are now we have some very odd and peculiar neighbours, already caught the husband in the back garden helping himself to our wood, we are the only two houses for miles but they make it feel like we live in the middle of a town, he banged on my window once and stood outside waving his fists at the dogs, boy did he get a mouthful and one hell of a shock as he thought I was out, I tend to pull the curtains day or night when I go out because he is such a wally!
To me it doesn't sound like separation anxiety more like stress at that moment, if she gets separation anxiety it would be all the time and as you say she is fine when you do it most of the time.
If she were mine I would leave the lights on, shut her in a room or confine her to an area where it is well lit, curtains pulled and she feels safe and happy, I leave the TV on but always check it is suitable programs!!!!!! By that I mean no films or programs that are likely to have strange nosies in or dogs barking, that always sets Summer off as she is a big TV addict and watches programs from start to finish, Midsommer Murders sets her off too as she reacts to the pheasants flying and foxes screeching which they always have in each episode, something boring would be the best trick!!!! Too clever for their own good they tend to think themselves into a tizzy sometimes. I probably wouldn't feed until your back unless she has had a chance to go out after her tea and do her business. Hope this helps and keep me posted.
