The SS only protects the POTUS, VPOTUS, their families, certain foreign dignitaries and other people in the line of succession. They may also take the security "lead" in other high profile national events that may or may not be related to a Presidential visit - a la the 2002-2005 Superbowls.
Agreed - I’ve been away for over 4 years now, but I cannot think of any reason SS would be doing protective details for SCOTUS...Even if hypothetically a joint POTUS/SCOTUS event were to take place off fed property, protocol requires your own individual (Or Agency) protective detail be with you. Facility protection may fall to someone else, but not to personal protection.