Capt Patterson also speculated (Appendix B) that the yellow object he saw might conceivably have been a large balloon or similar.
Many types of balloons, dirigibles and aerostats, piloted, remote operated, tethered or freefloating, ranging from a couple of feet to hundreds of feet in size, continue to be developed and flown by companies and organisations worldwide for many uses including military surveillance, weather recording, scientific research, advertising, sightseeing and cargo lifting n1 https://aiaa.org/pdf/inside/05_TC_Highlights/aiaa-lta.pdf; https://www.centennialofflight.gov/essay/Lighter_than_air/LTA-OV.htm . They are used in all parts of the atmosphere from near the ground to the stratosphere.
Meteorological balloons or other scientific research balloons released or tethered in the area would be subject to NOTAMs and other arrangements as indicated in Section 6h. The CAA Manual of Air Traffic Services n2Ibid. states that military operations or private piloted flights by balloons or dirigibles are subject to the usual clearances and Coordination Notices applying to any flights. We have found no evidence of any such activities. There is at present no clear radar evidence indicating the presence of unusual air vehicles of any kind n3 LTA gas envelope fabrics are not as a rule conductive and therefore are not radar reflectors (although some small instrument calibration balloons are metal-coated for this purpose). But other parts of the bracing structure or payload of a large LTAV may well be.. It's always possible that balloons or other types of LTA construction released at distant sites could become lost, develop leaks and drift into an Air Traffic Control Zone, but this seems a most unlikely theory on various grounds. The low-level winds suggest that balloons or stray aerostats/dirigibles arriving in the sighting location (ex hypothesi, having attained approximate neutral buoyancy at about 2000ft altitude) would have drifted directly over or at least very close to Guernsey with potential visibility from the ground by meteorological and other observers for a total period of perhaps several hours. It seems highly unlikely that such an event would have gone completely undetected. This is especially the case given the extremely large sizes implied in this case (several hundred metres) and the brilliant colouration reported especially by observers located to the north. It also seems extremely unlikely that such spectacular vehicles, having come nearly to rest at low altitude in the Channel Islands area, would then be able to depart the area at wind speed unobserved whilst Air Traffic Control is actively alerting air traffic to be on the look out. Or if they came down in the sea or on land it's hard to imagine them escaping discovery. And in either case it's scarcely plausible that stray equipment of this scale and novelty would have gone unclaimed by the operators who would no doubt have been searching for it.
A number of other objections could be raised but the above seem sufficient.
Plausibility: (0-5) 1