[zubi]

As a response to those who asked questions here in the comments:

When I finally overcame the shock, the very first thing I did was to ask her if she, or a guest, indeed searched for, or otherwise "input" anything related to usb-sticks or water-proofness into a computer system. The answer was no.

I don't know what to make of it. I don't want to succumb to conspiracy theories of the sort that ambient sound is recorded by our devices at all times. Most plausible explanation is perhaps the one about cognitive bias.

[bellyfullofbac]

An explanation would be that, in the beginning of Covid, your scenario (needing washable USB sticks) happened to many many people.

[zubi]

Excellent reasoning. I never thought of this but very plausible indeed.