"The report states that Muggles have been sighting flying objects for many years, unaware that they are observing escaped Quidditch balls. Professor Penrose recommends an increase in the fine for improper storage of Quaffles."
-- from the Daily Prophet article "Muggles not as Stupid as We Think, says Ministry report" (DP1)
Unidentified Flying Objects – Muggles do not realise that these are escaped Quaffles (DP1)
As reported in the Daily Prophet, a Ministry of Magic committee headed by Professor Phoebus Penrose drew attention to the Muggles thinking that they are seeing the flying ships of alien beings in the sky, which are in reality loose Quaffles. As Muggle awareness of these "unidentified flying objects" (UFOs) was a problem, "A Study of Muggle Suspicions About Magic" recommended an increase in penalty fines for allowing Quaffles to escape. (DP1)
While the date printed on DP1 is 31 July 1998, the timeframe for these events is 1992-1993.