The reason for the calls is the Verhees Delta, a one-man light aircraft which, as the company says: "looks small but has the widest cockpit of all light aircraft; looks only suitable for flights around the airfield but is in fact a very comfortable travelling aircraft".

Above all, the Delta is silvery and triangular and, because Verhees flies by day, gives off the occasional (this is Belgium, after all) glint of reflected sunlight. You can imagine Limburgers looking up and thinking they were seeing the advance shock-troops of some intergalactic mother ship.

The Verhees Delta, according to Peter Kuypers, a commercial pilot for KLM with experience of everything from gliders to a B17 Flying Fortress, "would be a good machine for flying cross country."

On 5 April, 2009,, at Beerzel, Antwerp province, at around 15.30 on a sunny afternoon, a witness saw a triangular object with a fin, of a light grey metallic colour in the sky, flying in front of a normal sports aircraft during a period of about one minute. The witness, who had had a keen interest in UFOs since the age of 15, was interviewed in August, 2009, by the UFO website ufomeldpunt.be.

In a 30-page report later published on the site, investigators report that the "object" could have been the Delta built by Bart Verhees. On further examination, that turned out to have been the case. "On 5 April I flew from Leopoldsburg to Ursel, most likely over Beerzel," Verhees told UFO investigators.

Since then, other spotters have reported seeing an object similar to the Delta, near Geel in the summer of 2009 and in Zonhoven in February 2010 - both in Limburg. The Verhees Delta is not going unnoticed.

Does Verhees himself think his plane looks like a UFO? "Well, what does a UFO look like?" he asks in return. "Before I flew the Delta, I didn't even know the UFO site existed. But now I'm not surprised when I come back from a flight and get an email asking if it was me. Sometimes I go onto the site myself to see if anyone's spotted me."