Earlier, as a coal train was going by, there was what sounded like a gunshot right by the kitchen door earlier. Something fell to the floor with a little metallic "ting."
I was standing on the other side of the kitchen, and I froze for a couple seconds, examining everything for bullet holes. Seeing none, and no one outside who might plausibly have been trying to murder me, I went over and picked the metal thing up. It was a tiny metal disc, not warm or scorched.
It had landed right next to the garbage can, which was roughly where the sound appeared to have come from. I looked in, halfway expecting to see an oppossum in there. (This wouldn't be the first time, okay.)
The culprit was, probably unsurprisingly to other people, the other half of a watch battery - I'd replaced the one in Buoyancy's toy bug for him this morning. I'd dropped it on top of a flat piece of dry cardboard which was at a slight angle, and it had slid down and touched the side of the garbage bag, which was also dry.
So, the train went past, the can vibrated, and the cardboard rubbed the battery against the bag, resulting in static, which made the battery blow up. Mystery solved!
This would have sent me into a panic spiral lasting for hours last summer; this time, even though I'm a few notches tenser than my norm, I just threw the rest of the battery away and went back to frowning at Photoshop. Good work, brain meds.