Battery now runs down quickly despite no changes

After almost 2 years of life, my iPhone 6 battery now runs down from 100% charged to very low percentage or zero every day, even with almost no usage. Took into my local Apple store, had it looked at and tested by 3 different techs, none of them could find any reason.

They tested the battery's ability to charge & hold charge and found it to be at 92% of it's original 100% when new, very good. We retried all the standard "turn these battery hogs off" like location services, auto refresh, closing all open apps, etc. Besides, I keep all those off, and also keep wifi off, almost all the time anyway, while making as few as 3 short phone calls and maybe 6-10 quick texts a day. That, plus the occasional photo and the occasional Google maps for directions, is all I use my iPhone for. It's the same small amount of usage I've been doing for 2 years, and up until a week ago, I generally got home late in the evening with still 75%+ of my battery still left. All the sudden, it drops precipitously every day with no change in my usage or what's turned on.

Anybody know what the lame techs at Apple couldn't find?