Secret Service had to know. Not their job to speak out. I’d have to think someone at DOD knew as well, and that there’s a procedure for his absence. But his top aides only seemed to know the tasks they were to take over, not his location. I’m sure they could have found out, which takes time. But this administration, who knows.
The Secretary of Defense travels with and is surrounded by DOD security and communicaitons staff 24/7. Him going by ambulance from his home to Walter Reed in Bethesda had to involve not only the ambulance but several Suburbans with lights flashing. His security is not Secret Service but instead elite DOD people out of the CID world, I believe. In any event, he travels a bit like a pharaoh. Not only shifts of security and other aides but also a bunch of people at Walter Reed had to know he was there. This was no unintentional oversight; it had to result from his express instructions. He went so far as to hold conference calls from the hospital, including it seems the ICU. He had to be on painkillers and very likely at some point was sedated.