picking up balloons (geese, parachutists etc etc)... can be done, some radars are better than other, and it also depends on performance on that day, affected by atmospherics, weather and equipment is set up.
If you recall old "steam-drive" wireless...(?) there were days when the reception was loud and clear, but at othertimes it would drift in and out or there'd be some static. In my experience radar works in much the same way.
Will add, crucially, these targets often generate very small weak returns, so another factor is the controller and skill to be able to pick this pin-prick out from other radar clutter (maybe experience, alertness or workload affected). If you are lucky enough to have clutter free radar, then you're probably not seeing these small targets. You can't win?
(Off course if you're required to carry Mode S when mandated the whole situation will change)