I had a barely used first generation iPhone in my safe and went to get it the other day to show it to some friends just to discovered it had blown up. It didn't look charred as your modem but it definitely exploded. Next day I ordered a few fire retardant boxes where I put all of my lithium batteries and power banks.
The problem is that there is plenty of stuff that you probably don't even realize has a Lithium Ion battery in it. With a phone at least it's obvious. But what about that controller for your e-bike? Camera? Remote control? Headphones? Authentication token? Toy? Microscope? and so on... they're everywhere. And they are not always announced properly, especially smaller pouch cells.