Thank you for your reply.
It's actually a very interesting question, and a relevant concern that a few datacenters have raised for me. For the data centers who do not want their location publicly available I have made it possible to be listed in a special way - see
SpringNetl as an example.
My personal theory is that it is more or less impossible to keep the address of a co-location datacenter 100% secret, specially if it is a big one.