ORIGINAL: Silent-AV8R
I disagree about the BEC. It is very possible that the BEC or ignition was causing an internally generated noise that would not result in a failsafe event. This has been documented before. Failsafe works if the RX loses contact with the TX. But internally generated noise from a BEC issue, ignition, bad servo, bad connections/switches are a very likely culprit in this case.
You really pinned it down didn't you?