Why did the Test Day end with a red flag?

A few minutes before the end of the Test Day, Bruno Vandestick announced that the n°7 Toyota - the fastest car yesterday - was in trouble at Mulsanne. The red flag was waved and all the competitors set off at 80 km/h to reach the pit lane.

The security cameras then showed the Japanese car going around the Mulsanne traffic circle and setting off again at low speed towards the Indianpolis bend. There was no need for a red flag. But why was the red flag waved?

A few minutes after the chequered flag was waved, we ask Pascal Vasselon what had happened a few minutes earlier. 

"We had planned to do an exercise with the race direction and the stewards, when they have to intervene on a car with a hybrid problem," explained the technical director of Toyota Gazoo Racing. "Just to do better than what happened. But there was a misunderstanding between the race direction and us."

Just that...