Large fridge shaped object, observed for 10 minutes, moving slowly at low alltitude over populated area during morning rush hour.

I was driving to school heading eastbound through farm land from St. Catharines to Virgil In the Niagara penninsula. It was a clear sunny morning in June. The sighting began when I noticed a fridge shaped object hanging in the air appearently motionless over the Niagara municipal airport. I stopped the car immediatley. I was about 1km directly north of the airport. The object appeared to be approximately 60 feet tall and perhaps 20 feet wide. Sort of a rounded rectangle, it appeared to be grey in colour. It couldn't have been more than 150 feet off the ground. At first I thought it may be an unusual hot air balloon. Except it had no basket at all. I then noticed it was moving very slowly away from my position to the south. I watched in amazement for about 10 minutes as it slowly disappeared over the southern horizon, the Niagara escarpment. It's course would have taken it directly over the Queen Elizabeth Highway, a major artery for traffic through tthe region, no doubt very busy at the hour. I called the airport later that day and was told there was no balloon activity in the area that day .
Date is approximate.  PD