David Rudiak:  
 A Blue Book unknown case also occurred that day in Fort Meyer, Florida  where a 75 foot saucer was seen through binoculars by a former aircraft purchasing agent.  Ruppelt wrote of the first UFO/radiation detection case at Los Alamos in Dec.1950. From  Dec. 14-20, FBI memos mention three more UFO reports by radar operators and employees at OAK RIDGE. Two jet intercepts were attempted and on 12/14, "a group of objects blanketed the Radar Scopes..." for 3 hours. These incidents had been preceded by a flurry of 15 such UFO reports back in Oct. over Oak Ridge, also investigated by the FBI. The one that occurred on Oct. 23 is also mentioned by Ruppelt in his UFO radiation chapter. One FBI memo stated "Geiger counters in the vicinity [had] unexplainable  readings at about this time." 

For all we know, this may only represent a fraction of UFO  activity at the time. The UFOs over our atomic installations like Los Alamos and OAK RIDGE must have been the most alarming. Maybe the Dec.6 incident really was a false alarm, but the  military in conjunction with the other UFO incidents were concerned that we might not be able to distinguish a genuine Soviet attack from the UFOs that were flying around. Perhaps this triggered the alert on Dec. 8 for any additional information on the UFOs. It could have been fears of an accidental nuclear war with the Soviets at the height of the Korean War.