How much of a geriatric check up was done? did he have bloodwork done? was a stool sample checked?
He could have been fine last week and still be sick today and vet offices are great places to pick up infections.
I think a 'nanny cam' set up would tell you a lot. This could have been a one time thing because something loud or scary happened.
You could try leaving a tv or radio on low (no talk shows pick something soothing!) to see if that helps.
You could use a baby gate to confine him to the room with his pee pad and a comfy bed etc. when you leave so he can't get in trouble.
You could leave him a select laundry item likean old t-shirt you've worn to be 'comfort' while you go out.
You can give him something such as a stuffed Kong toy to work on when you leave to keep his mind off the fact he is alone.
and you can read here for standard treatment if it is separation anxiety
http://www.usask.ca/wcvm/herdmed/applied-ethology/behaviourproblems/anxiety.html
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