I've had pilots report spurious RAs both on departure and on arrival close to different airports over the years with nothing seen on the radar (nor out of the window). I have suspected TCAS interogating itself via ground reflections. Others could be against aircraft alreading squawking on the ground as happens more often in these days of A-SMGCS. Either way I have filed the appropriate reports so that minds immesurable superior to mine can regard them...