I started wearing my chin strap consistently due to other reasons and found that my 95% leak rate is consistently zero. In fact I fairly frequently get 4 zeros for leak rate.
My take away from this experience is that I have been causing leaks with movements of my mouth. I know from testing that i can cause a leak in the lower portion of the mask by various facial movements like a wide grimace.or a big yawn. Apparently the chin strap reduces my facial movements during my sleep. I have a definite love/hate relationship with my chin strap but it has proven its value in my case.
If you can not feel any leaks when going to sleep or waking up and at the higherst pressure used then the leaks are probably mouth leaks of one sort or another.