Long pole on the tail of a Cessna, what could it be?
What you describe does not match any banner tow rig I am familiar with, and I have towed banners.

What it sounds like is a magnetometer, which might be on a survey aircraft. They can be on a boom (stinger) or sometimes towed.  I have seen them on Cessna 182, 210 and 208 aircraft, as tail mounted booms.

Magnetometers are typically used for geophysical survey work.  Towed magnetometers may be more sensitive and might be useful for detecting submarines, mines, etc. The P-3 typically has a rear boom with a magnetometer or magnetic anomaly detector (MAD).