Optical sensors would probably detect it
A old GEODSS sensor can track a volleyball-sized object out at geosynch. The sensors have been upgraded since then. It's a lot harder to hide an optical signiture than a radar return.
There are other weaknesses in the scenario, such as the need for TT&C (telemetry, tracking, & commanding) means you have to have antennas on the vehicle. The distances involved mean you'd either need extremely powerful ground transmitters or high-gain antennas on the vehicle. High-gain antennas are hard to hide, RCS wise.