Here is a good one. I used to have a Fritz!Box as my router. This was replaced a couple of months ago with a Synology router. Regardless, I have been using Cloudflare Zero Trust DNS for years now and noticed an interesting thing happening: thousands of DNS queries on subdomains of fritz.box as per the screenshot below.

These domains don't exist. They are subdomains of Synology, Microsoft and Backblaze. I suspect these queries are coming from my NAS - these three services are used in the NAS. These queries don't seem to impact normal operation as all services still work on their default domains (microsoft.com, backblaze.com, etc).

I wonder if the NAS was assigned a name in the fritz.box space before and kept using this when querying and keeps trying these.