It could well be co-location issues with other circuitry, but am wondering if anyone else has had this specific issue, which so far has proven very difficult to track down…
The spurious transmissions look to be happening when there is either:
no WiFi
WiFi but no internet connection
Both of the above tests were performed in safe mode, so our application is not in the equation.
As we have lost access to our loan Spectrum Analyser, we will need to wait for delivery of our new one before we can fully answer:
Does the issue occur with internet connection (in safe mode)?
Good news is that we were able to resolve the spurious transmission issue within our application by calling particle.disconnect() if there is no internet connectivity.
I note that particle.disconnect() does not disconnect the WiFi radio (good).