I was flying with 15 gps signal and already calibrated my drone . After I decided to return back to home, my drone started loosing its flying height . I have never faced with such issue before. I am an advanced operator and caring all issues before I fly. There was no harsh wind or weather conditions etc. My drone was calibrated and battery was full and seems ok(total charge 14).  What was the reason that my drone loosing its height itself?

Ive experienced the same with mine.
I let it hover over the ground for a minute or so when i suddenly realized that it was loosing height, although very slowly.
Could be an error in the heights sensor would be my guess as i was only about 5 meters above the ground.