Can a drone be seen on TCAS?
No, a drone will normally not be visible on an aircraft TCAS. The TCAS system is based entirely on direct communication between aircraft transponders, so both parties (aircraft and drone) must have a transponder in order for TCAS to work. Normal drones do not have transponders, so they will not be visible on TCAS.

If a pilot reports a drone near the aircraft, what he means is that he (visually) has seen a drone out the window, not that it is indicated on an instrument anywhere. Besides, TCAS does not know different aircraft types, so even if a drone had a transponder and it was presented on a TCAS, the pilot would have no way of knowing that it was a drone (as opposed to any other aircraft).


  Or just primary radar target pick up by TCAS. 


TCAS is NOT a radar. It is not able to display anything without a transponder. And even ground based primary radars will not necessarily pick up a small drone.