You probably need two turns of the Radar PPI tube to verify that the signal is lost, or has a weaker echo, by losing a lot of height quickly...all these mean a weak Radar return signal, if the Plane is in trouble.
Radio is the first and best communication until plane nears Airfield. The Radar at the Airfield may not have even bothered to pick up the echo...but generally relies upon radio contact until the plane is closer in.

Similar in ships...The Coastguard Radar will not spot the ships transmitted Radar pulse ...if the ship is angled, that it might sink. Its Transmitted pulse gos up into sky or down into the Sea.
Again Coastguard Radar would need two revolutions of the Radar PPI before verification.