Pretty low risk for an old cell phone. The main initiator is an internal short circuit that allows the electrochemical release of stored power into some tiny spot in the cell. Old cells eventually self-discharge and remove that energy release as an option. The usual cause of that internal short circuit is the growth of lithium metal dendrites that pierce the insulator membrane and that most often grows during recharging so also a limited problem for stuff stored in junk drawers.
A stupid cause was by Samsung. While the write up in Wikipedia lists other causes (
the contemporaneus coverage was that the cells they used for a cell phone (no relation) would fatten up slightly when they were charged. Unfortunately they had cells that were something like 0.1 mm too thick for the gap inside the cell phone and this put a large amount of squeeze on the little foil pack which got carried into the phone where the internal terminations were. I am unsure if that damaged the termination so that the charge current went through a smaller section due to ripping the termination or if it forced a hole in the insulator. They recalled millions (I think that was the number) of phones as several had already cos-played Johnny Torch on nightstands next to sleeping owners. I don't recall any deaths, but it was only a matter of time to reach certainty. What they said about welding and so forth in the Wiki article could be "fixed" by simply providing a battery replacement, but they didn't, indicating that it was the structure of the cell phone which they could not change that was a key issue. Anyway, some stupid causes.
The final one is the stupidest. It's been a while since vaping got crazy, but there were some trying for what appeared to be megaWatt outputs to challenge coal-rolling truckers using vape pens and some of these guys would put spare li-ion cells, without power limitation circuit caps on the ends, into their pockets along with spare change and keys, producing an external short circuit. Use protected cells - there is an attached circuit board, and don't put them into pockets or drawers with other loose metal items.
There are always the intermediate failures, phones spontaneously going off in pockets, but I think most of those are a variation of the Samsung squeeze and a bloated pack that probably made a noteworthy bulge in the shell of the phone. The laptop issued by my wife's work did that - it got to where it was a large radius spherical section; didn't rupture but it did get removed from the charger and taken for replacement right away.
Oh - a complete left turn on this. Again, a charging issue. At the start of the Gulf War the US Army made a lot of use of a device called a PLGR (Precision Lightweight GPS Receiver). The planners knew two things about this device. One is that, at the time, they would not be used at all, sitting in storage awaiting the call. They also knew that when the time came there would need to be reliable power for them and the users might not have access to external power. Smart people they were they created two battery packs. One use lithium primary cells and the other used lithium rechargeable cells and of course they are interchangeable in the PLGR. They also included the ability for the PLGR to be run from a vehicle power supply and recharge the rechargeable pack. This meant that lithium primary cells, which have a decade long shelf life, could be dropped to forward patrols and used immediately. This was the main inventory as the rechargeable ones didn't have a great shelf life/weren't in large supply - not really sure but they were relatively new.
The problem was that the designers did not make it impossible to put that device in a mode to charge a primary cell. Every so often someone would find a dead lithium primary cell and go to charge it up and it made it into a fire grenade. Since these PLGRs and their batteries were critical to an operation in a open desert with no distinctive land marks, this happened enough.
One of the contracts my company had was fitting PLGRs along with a bunch of other ELINT/SIGINT equipment into HMMWVs.
As an aside, it is my understanding that there weren't nearly enough PLGRs to go around for the Gulf War and so soldiers were asking family members to go and buy the sporting goods GPS navigation units and ship those. Rumor was the Arabs were completely taken off guard because they had never encountered a force that could organize across vast stretches of desert where they had experience in reading the landscape.
Back to the old-stuff storage - an area that should be of great concern is that electric bikes and scooters and so forth tend to get accumulated by humans thinking they can refurbish, repair, strip and sell parts, or are just obligated to collect fleets that landfills won't accept. There might end up a warehouse with 10,000 of those rental scooters owned by a company that has gone bankrupt.
Imagine this, but with batteries:
