"SunnyvaleSite" is correct, and adding that as ad_site is what fixed (or
worked-around) the problem.
what do you mean by "did not try to use that when it could not
look it up while online" ?
When I was online (without ad_site in sssd.conf) the log showed the "Could not
autodiscover AD site" messages above, and there, or just after it, the attempt
When I was offline then it showed the "Looking up domain controllers in domain
and site "SunnyvaleSite" (with that DNS lookup failing since
I was offline) before it logged me in with the cached info.
So somehow it knew the site name without being able to look it up online.