Their small size alone makes them stealthy.
The wings & body are composite, which doesn't reflect radar energy.
The engine is hidden and not exposed.
It can fly as high as 9,000 feet, making them impossible to acquire visually.
They orbit at approx 50mph...doppler radars will not display them.
My guess is the best way to locate them is to triangulate their radio transmissions.
While true, doppler processing isn't the only mode on a LOT of radars. A coherent monopulse, multimode/multi-PRF radar can lock on a nonmoving target quite quickly. In the ideal case, a radar monopulse comparator (Sum, Delta Az/Delta El) can see and angle locate a target with one pulse