By Alexa rating is 7th most frequented Google subdomain mu-in-f104.google.com. Do you have any idea what it could be?
alexa.com/data/details/traffic ...

There are a whole bunch of these following the same format. I started adding them to my subdomains page but then couldn't be bothered because there were so many:

ruscoe.net/google/google-subdo ...

I first came across them when I did a reverse DNS lookup on the IP address used for ghs.google.com. For example:

   * ghs.google.com (CNAME for ghs.l.google.com)
   * 72.14.207.121 (IP for ghs.l.google.com)
   * eh-in-f121.google.com (reverse DNS entry for 72.14.207.121)

All seem to follow a similar pattern but not all of them point to the Google Apps custom domains address.