Actually I have similar question, and frankly I have not yet found a definite answer.
My Xiaomi 10500 mah always drains itself unused in storage. From fully charge, the indicator shows only 3 blinking led after just 4-5 days, 2 blinking led after about 2 weeks, 1 blinking led after about 1 month. I have tried to research about this, and it seems some powerbanks never actually turn itself off in storage. They just seem to fall into sleep mode, ready to charge immediately as soon as it detects some demand from the USB port. And that is why some manufacturers have what the call "Standby time".
The drawback is , the powerbank constantly discharge itself during standby / sleep mode.
I really hate this Xiaomi powerbank.
I only use powerbank once every 1 or 2 weeks. And everytime I need to bring it with me, the powerbank capacity only shows 50-75% capacity left because it drains itself in storage. It beats the purpose of buying a 10500 mah powerbank when everytime I try to use it, I only have 5000-6000 mah left in it.
My previous powerbank did not do this !!!
It didn't automatically charge when I connected a phone with charging cable to the charging port, but I had to physically pressed the power button to wake it up and start charging the phone. It didn't lose its charge during storage, even after 2 months in my drawer unused, the powerbank capacity indicator still showed fully charged when I turned it on. The only little inconvenience is that you have to press the power button manually to wake it up & start charging the phone.
Now, the question is, do we actually have 2 kinds of powerbank based on how it operates ? One that only sleeps to automatically start charging a phone , and another with manual power on ?
If yes, it really bugs me to see no manufacturer mention anything about this fact.
Note: the power button in Xiaomi 10400mah powerbank does not seem to be used for Power On/Off the powerbank. It doesn't actually turn off...only sleeps. The power button seems to be used only for the purpose to check the battery capacity.