Thanks for the endorsement
I send/print this handout for people as a primer on SA.
First you need to figure out if it is SA and not boredom. Some signs include sweaty paw stains, drool, or hyperactivity upon your return, just to name a few.
If it is SA, this handout talks a bit about what to do. But, the big thing to do is planned departures. Short little practice departures where you return before your dog gets upset. Over, and over, and over again, with increasing duration.
www.ddfl.com/behavior/sep-anxiety.pdf