Alex, first of all, yes this ping is a SIP OPTIONS sent to the endpoint by the Sofia SIP stack.
I'm not sure about the (unknown), I have always assumed it to be a reference to presence status. The information on this Status->Registrations page is gathered from a Freeswitch event socket request. You can get the same information at the fs_cli with:
> sofia status profile internal reg
The event socket call actually uses statusxml but the data is the same.
The reason I made the assumption about (unknown) referring to presence, is because, if I take a user (201:mydomain.com) from my registrations list and use it in the following fs_cli command:
> sofia_presence_data status 201:mydomain.com
Because we seldom use presence, it always seems to return "Unknown".
I may be wrong about the presence thing, but you are safe to ignore the (unknown) in the registration status, it is nothing to worry about.