The South Wales Police while out on patrol in a helicopter over the Bristol Channel on 9/24/2016 spotted an invisible UFO. This is not a contradiction or joke. The helicopter crew was night flying about 1000 feet above the channel and was unable to see anything unusual visually or with their normal helicopter camera. But this crew had an infrared camera on board. The camera was able to pick up a heat source flying out over the water. If not for the infrared camera, the UFO would have been unseen.
This case shows an aspect of UFO Sighting watching that goes unnoticed pun intended. UFOs have so many times been spotted and then just disappear. Sometimes they fly off at incredible and unseen speeds like a "Tic Tac" in a Navy UFO sighting, so fast that they are beyond the capability of the eye to register and sometimes, at night, they just disappear perhaps by turning off all but infrared light. The ability to "disappear" has been pointed out by the Navy's U.A.P.T.F as if it were a recently discovered unique characteristic of U.A.P. Actually knowledge of this particular characteristic goes way back to the summer of 1947. The object filmed on the infrared camera was round and gave off heat. They watched the object move against prevailing winds and it was obvious that there was an object: it gave off heat and it was not moving with the wind. This UFO Sighting report abstracted from a forthcoming book that reports sightings by police in the USA and the world over.