What's a long-standing mystery that still preoccupies you?

In the depth of pandemic lockdown, after my roommate moved out to return closer to family, I was in my house alone for a month straight. One day I hear the tea kettle whistling on the stove.

It was the middle of summer, I hadn't made tea in weeks. Maybe I bumped the stove control? But there shouldn't have been any water in the kettle. And I hadn't been in my kitchen for over an hour and it wouldn't have taken that long for the water to boil had I put it on and just forgotten about it somehow. I keep my doors locked.

Idk, the only thing I can think of is the isolation really got to me that day, I put the kettle on and completely forgot I had done it five minutes later.