Here are notes on the CW radar measurements in 1985 updated in 2013. echo.jpl.nasa.gov/asteroids/Sisyphus/sisyphus.html
They could not detect it with the ranging technique, using a single wavelength and timing, but the continuous wave showed a spike off center that changed in relative velocity. After 2000, there were several binary asteroids detected in CW radar by noting that the velocity period of the secondary was different from the rotation period of the primary. They then understood that 1866 Sisyphus was a binary.