The runout sensor stuck to TPU a lot more than PLA, but it was still better than a bowden tube.  The search is on for a less grippy string.

The alarm got a satin LED to try to make it more visible.  Sanding down clear LEDs never ended well in the past.  The problem is getting an even coating on the rounded surface.  This one took a few hours.  It would never be practical to sand more than 1.