The watering can was only the latest example, but it's as good a place to start as any -- until the next thing turns up missing (yes, I know those last six words were an oxymoron):
A few days ago, during a heat wave, I carried a watering can to the Common Area in the trailer park I live in. I usually rely on a water barrel that's there, but I knew it would be empty, or nearly so. I did my watering, and then got into a conversation with someone. I forgot about the watering can and wandered off to another part of the park. Next day, there was no watering can in its usual spot on my porch. That's okay, I have a spare, but as the days went on I began to worry that my evil twin might misplace that one as well, and then what will all the poor dry plants do? <cry> I started looking for it, but it was nowhere to be found. :-(
Yesterday morning we got almost an inch of rain. Maybe some of it seeped into my brain, reminding me that I hadn't looked in the Common Area. I checked there, and <eureka> there it was. The woman across the street started razzing me about watering the plants on a rainy day.