The teacher's face seemed strangely familiar to the researchers. Over a few weeks they noticed him reappear in different schools - always on inspection days.
It was one of the clues that led academics from Manchester university to conclude that significant numbers of schools are "borrowing" impressive teachers to get through Ofsted inspections.
Their discovery came after visits to deprived inner-city and rural schools in more than 30 local authorities in England.
Professor Mel Ainscow said: "We have seen this going on for four or five years in a fair number of places. It is a sort of corruption that has become endemic."
He believed it was a result of the pressure from the Government on schools to raise standards. The same pressure was responsible for schools excluding difficult pupils or putting them on the special educational needs register before inspections - practices also uncovered by Professor Ainscow's team and revealed in The TES earlier this year.
"It is the dark side of national policy," he said. "It is not the instinct of schools to do this. But once you know that other people are up to this kind of thing, you are tempted to join in."
The practice had been made easier by the growing amount of collaboration between schools, encouraged through programmes such as Excellence in Cities.
Professor Ainscow's team concluded that increased collaboration was generally positive for deprived schools. They found it allowed them to help each other solve problems with staff-pupil relations, to raise the expectations they had of their pupils, to develop their curriculum offer, and to address better the needs of vulnerable pupils.