If it were me, I'd try to follow the wire to see where its been routed and what its connected to. From memory, if cruise control has been retro fitted, it is possible that someone may have taken a power feed directly from the fuse box like this. I think there are threads and guides on this forum about this very subject if you search.
I had a Loose Connection error message display a few weeks ago. Shortly after the engine (JTD) just stopped. Complete loss of all power. When I tried to restart it, there was no sign of any life. Initially I thought it may be immobilizer related. After 10 minutes or so, It started straight up, and drove perfectly, then after 15mins of driving, it stopped again. I did a bit of research on here, plugged in a diagnostic scanner, and found it was the crank sensor. After replacing this, it is now fine again.