My estimates are based upon personal observation over the years. They are in fact a bit low compared to the amount of cells I have had leak both in and out of lights and stuff. I would bet many here would probably more agree with my numbers than 1 in 10 million.... else there wouldn't be constant talk of putting lithiums in devices that are going to sit for more than a week anywhere.
Where did "1 in 10 million" come from? I stated 10s of parts per million, or in other words, on the order of 1 in 100,000. These numbers are based on rates of consumer complaints in the industry as well as years of leakage testing data (undischarged, partially discharged, over discharged, high humidity, room temperature, elevated temperature, thermal cycling, etc.).
Also, I would think that the OP of this topic was asking about storing undischarged cells, which are less prone to leakage, alkaline or otherwise.