Perhaps it was crawling a list of IP addresses and your web server is also serving the website to your IP address (not domain/subdomain). You can configure the web server to show blank page or 403 error if accessed by IP address.
Seems like a promising concept as real alternative to Google/Bing.
Initially I couldn't figure out how to spell it and this doesn't help to grow the branding. Apparently it's pronounced "mumble", so I recommend they just renamed it to mumble for better brand recognition. It's way easier to spread the word.
Perhaps it was crawling a list of IP addresses and your web server is also serving the website to your IP address (not domain/subdomain). You can configure the web server to show blank page or 403 error if accessed by IP address.