Another great premise for a mystery novel

I was compiling a spreadsheet of all our org's costs associated with printing in the last four years, so I had to go into storage and look up our old bills paid. On the shelf with all our accounting paperwork, between the two binders of bills from 2020, was a third binder without a label on the spine, so I pulled it in case...

... and it took me several minutes of flipping through it to realize what it was: it's all the bills and receipts from a vet practice I've never heard of in a nearby town, also from 2020.

You might think it would be an easy mixup for an accounting firm, but we use a local business college and they don't make visits like that. They wouldn't be in the document storage at all: we package our documents up and hand them over in envelopes, and that's how we get our bookkeeping reports back from them. It's just the volunteers and me (or my predecessor) in the document storage room.

(To make a good mystery there would have to be some further drama about the binder, but it's a good setup.)