At first I thought it was a password. Or a flight number. Or maybe a forgotten account ID.

Or maybe — and here’s where it gets weird — it's the key to an encrypted message. The notebook's other pages are filled with seemingly random numbers. But every 8th line ends with a four-digit number that feels... intentional.

But then I ran it through a hex-to-ASCII converter just for fun.

Tried it as a hash fragment. Tried it as a serial number. Nothing.

I found this code tucked inside an old leather-bound notebook at a flea market — no name, no date, just ca857e71 written neatly in the center of the first page.