I have no public numbers but the learning materials for fire safety at the work place here has lithium ion batteries from old devices called out as the most common reason for fires in office buildings. Likewise the waste management companies here are routinely reporting fires in containers because people throw away old devices there against regulation.
That's not quite the same thing as sitting still in a drawer though. Lithium-Ion batteries really don't like to be dropped or crushed. Basically anything that is a threat to the mechanical integrity of the cell should be really avoided.
Not that I'm aware of, and I'd like to find some. It's a known fact that LiON batteries can spontaneously combust. I wish I had an easy metric which helped describe the risk of keeping old devices around.
When I relegated my n-1 iPhone to driving music on my stereo I discovered that n-2 had a badly swollen case. Around the same time, I discovered an old MacBook Pro also had a badly swollen battery when I went to remove an SSD from it I wanted for another purpose. It's not clear to me how careful one should be about policing this sort of thing in general. Certainly I'm sure I have old LiON in things at home.
