I’d be checking if the internet service has a static IP. If it does, and another letter comes respond to the letter with “I believe this letter has been sent to me in error. My Static IP provided by my ISP while being similar to the one listed, is not the same. If it’s not check with the ISP if they have a record of all IP Addresses used for that service.

I’d also ask for specifics when the infringement occurred and as much data on the machine like hostname or local IP used in question, there is a chance someone hacked the wifi.

Perhaps it was accumulated temp files and windows updates. Are you using OneDrive? Perhaps it’s cleared out a bunch of files you’ve not used in ages?