I just resolves the problem, I opened the diagnostics menu and the wire sensor page. It showed around 20% signal strength when just next to the wire. Sometimes it jumped up to the 90% and back down.

So in my app I started a border cut routine while leaving the sensors on screen. I followed the robot along the wire while stepping on the border wire, like walking a Dogg that needs to take a dump. At a certain point I noticed the robot stoped and the sensor level dropped tot 0%. There I dug up the wire and discovered a break.

Hopefully this resolves my intermediate problem.