Sensing motion of polypropylene twine

The application is tying round rolls of hay after they have been gathered and baled. When the baler measures a full bale, arms made of tubing with the twine fed inside them move into the bale while it is still turning and the twine (string) is picked up and placed across the baler. The string is Pulled onto the bale; there is no device that feeds or even comes in contact with the string with the exception of guides along the path from the initial ball of twine to the end of the twine tying tubes.
It is difficult to see the twine at all, aside from it moving because there is only a small area where the string can be seen. On occasion one or both strands of twine will get picked up are pulled and baled up throughout the entire bale which results in a bale that cannot be used and a huge waste of twine.
It would be ideal if there is a sensor that the twine can pass by or through, non-contact, that can indicate when the twine is moving so the sensor could turn on an indicator near the operator on the tractor.

The twine is made of polypropylene and is most often 20,000 foot balls that are about .110" in diameter. The color is usually blue but can vary from black or orange. Rarely an organic sisal twine is used which is tan/brown in color. The diameter can vary up to around .1875. This could be better controlled if diameter is a major factor in the measurement. I would say the string varies from shiny to more of a matte finish.
The only trigger would be the string itself moving, no other indicators that I can think of. Since this is an error in the normal operation the auto-tie indicators could not be used.
Has anyone worked with a similar measurement application? What kind of sensor(s) or other devices were used?
Thanks for any help.

Well now... there's an interesting problem.

If you could touch the twine, a small rubber wheel comes to mind.


If you can't touch it, about the only thing left is to look at it.
That kind of twine is made up of several kinda shiny strands.


So, shine a light on it.
As the twine moves past the sensor, it will vary in reflectivity.

It should vary enough to translate that variance into a "moving or not" status.

There's lots of different ways to show variance on your tractors console;
a indicator light, a meter, a single line LCD screen, etc.