I’m currently going though this with my soon to be 1 yr old boxer. She had a bladder infection but still once it has cleared has the incontinence. The vet done blood work and suggest kidney issues, as I am a Independent researcher of biology and nutrition I believe she is wrongfully diagnosed and am working on getting more test done (ie. xray, ultrasound, more bloodwork etc..) as my dog’s estrogen levels were normal I’m quick to say its not due to that nor infection so i may be from her genealogy, one cause is the tubes from the kidneys one may protrude the bladder and go directly into the urethra. As your dog is a little older before it started i would suggest getting a urinalysis to eliminate infection, then bloodwork to check her estrogen levels which when low cause the sphincter muscle to relax too much and cause leaking when relaxed/asleep and to test for other enzymes (phosphorous, protein, bun, etc..) which can signal kidney troubles. Most common cause is infection if not that most likely its her estrogen levels, its very uncommon for kidney issues to arise in such a young dog. One piece of advice, do not buy the crap the vet sells they call food for dogs with issues with kidneys and do not take whatever answers they give you without question, its a business, do your own research and determine for yourself.