I haven't seen anything like this on a 172.
A few years ago I was visiting an airfield, and saw and aircraft (cub or robin, can't remember the type for certain) flying the downwind leg with, apparantly a thin trail of smoke or oil behind it. I was on the point of going to raise the alarm, when I realised that it was a glider tug, and what I could see was the tow rope.
Brooklands