Keep this note with puppy barking when you leave (https://bookmarkextent.com/story24508606/save-tiny-goodbye-practice-steps). The note is titled “Save tiny goodbye practice steps”. It covers tiny practice exits. Then it points onward to an article titled “Alone-time barking and tiny goodbyes” and Love My Puppy's product page for the facts.
If leaving the house sets off barking, start smaller. Step out for a few seconds and come back calmly before you try a longer exit. Keep those first practice goodbyes short and boring.
The product page this note eventually points to is a rechargeable ultrasonic