We have billions and billions of old devices with ancient batteries laying around, pretty much every house in the developed world has at least one, more likely multiple, lithium batteries laying around dead for years.
There is no need to do research or dig into it, the experiment has already been running for years in every house in the nation, and random battery fires are still rare enough to be news worthy. If you find a forbidden pillow (swelled battery pouch), dispose of it, but even those almost never convert to fire/explosion.
A more responsible answer would've been something along the lines of: There is a very small chance, but if you take the little time to responsibly dispose unused batteries every once in a while, then you do not even have to think about this.
The chance is already so low that it is firmly in the "you don't even have to think about it" category. Stove tops cause 160,000 home fires a year, killing 135 people annually, but when was the last time you felt unsettled when looking at your stove?
Lithium batteries are just prime fear porn for the media to run with. New and scary technology. But the statistics paint a wildly different picture.
We have billions and billions of old devices with ancient batteries laying around, pretty much every house in the developed world has at least one, more likely multiple, lithium batteries laying around dead for years.
There is no need to do research or dig into it, the experiment has already been running for years in every house in the nation, and random battery fires are still rare enough to be news worthy. If you find a forbidden pillow (swelled battery pouch), dispose of it, but even those almost never convert to fire/explosion.