Autopilot taken over by "Smart Device"?

[ProjectManaia]

Hello everyone,
We have a Simrad Autopilot on board, that worked flawlessly for years.
It is a NAC-3 controller, hydraulic drive, has a Simrad heading sensor as well as a backup heading sensor we added as we realized people with cellphones sitting next to the existing antenna make the autopilot a little crazy. Anyways, that problem was solved. We also have the Bluetooth remote for the Autopilot which never gave us headaches either.
This year all of a sudden from time to time the autopilot simply goes in to NFU (Non Follow up) and goes hard to port. This happens both, with engaged heading hold, but also in the middle of the night on anchor when it is in standby.

My suspicion is that some smart device for some reason taps into the Bluetooth and doe this but I could not confirm anything. We have a crew of 10 on board that switches every two weeks and the problem usually appears for one of those periods and then all is fine for the other crew. So really I was wondering if someone had a similar issue and how you went on with tracking down the source of the problem.

All thoughts and pieces of advice would be wildly appreciated!