We adopted a dog at about age 3, who was house trained etc. We found out quickly he had separation anxiety.
He even chewed up the door frame while we went to the store one day. got out the CAT door { he is 70 lbs or so} when we left him in the garage, we thought we were going somewhere without him. He got out before we could get in the truck.
We tried leaving the light on, tv etc It took 6 months or so before he Outgrew This.
I have no suggestions just sympathy for you.