Sensors have an operating range. So it's possible that you have a sensor that is reading faulty (either higher or lower than it should be) but still be within that operating parameter. Therefore, no code is generated.

When you're towing, you're changing the operating conditions of the engine with a much higher load. If in these conditions, the NOx sensor at a particular time is supposed to be reading somewhere between X and Y value, and it pops up with Z value because the higher load makes it read OUTSIDE of it's operating parameter, it will pop up a code at that time.

It sounds like you have a sensor that is on it's way out, but so far only high-load conditions force its readings outside of normal range. At some point, it will do it consistently.