Gas and Oil Home Heating Furnaces - flames from chimney?

I have an 8 year old gas forced air furnace located in the basement of my 2 story 2,000 sq,ft. home near Chicago. A neighbor says they saw a "flame shooting out" of my 6" chimney pipe through the roof (I do NOT have a fireplace or brick chimney). Any suggestions to what it might have been? Something blown up on the roof that may have ignited? An overactive imagination of my neighbor? There is no smell of gas within the house. I am open for ideas. Thanks.

The only thing, other than seeing something that was not there, that I can think of which could cause this is a gas valve leaking thru & allowing gas to flow thru the heat exchanger & up the chimney. I guees it could ignite when the furnace came on. I would think you would get one heck of a bang. The exhaust of a furnace would not get hot enough to ignite a bird nest or any other combustable material.

If it is happening, it will keep happening. So you thank your neighbor very much and ask your neighbor to call you right away next time.

My guess in reflected red light for a couple of seconds as the sun was setting striking at the correct angle of incidence to make a bright flash in your neighbor's direction. If your pipe is still shiny, it could have been a car's brakelight or any of a hundred other things.