Google has added a new Google crawler, a new user agent, to the Google spider list. Its name is Google Site Verifier User Agent. This user agent ignores robots.txt rules, by the way, and I assume it is used for verifying sites with Search Console and other Google tools and services.
Google posted the new user agent over here in the Google Crawlers help documentation.
The user agent token is: Google-Site-Verification
The full user agent string is: Mozilla/5.0 (compatible; Google-Site-Verification/1.0)
And Google noted with a “caution” that the “Google Site Verifier ignores robots.txt rules.”
So keep an eye out for it in a very limited basis.
Forum discussion at Twitter.