I found the cause of the Electromagnetic Interference warning I was getting on my M2P remote controller. I received warnings inside and outside... whatever location I tried to fly in.

I bought a Radio Controlled, Citizen "Blue Angels" watch earier this year. It receives a radio signal from the atomic clock in Denver each morning, about 1, 2 and 3 AM. This signal updates the time on my watch. As it turned out, the watch was responsible for the EMI I was experiencing. I stopped waring the watch when I fly my M2P and haven't received a warning from my RC since. Who would've known.

Hello and good day walttheartist. Thank you for sharing this experience with us today. If you have any inquiries or concerns please feel free to post it here at DJI forum. We are here to help you. Have a safe and happy flying always.

There is an app for it, of course :-)
Most smartphones have a compass (and therefore a magnet), and various apps can measure the electromagnetic field quite accurately. Normal value is between 30 and 60 microTesla, higher than that indicates that there is some kind of interference. I use the "ultimate EMF indicator Pro" https://play.google.com/store/ap ... imateemfdetectorpro (for android)
Right now at my desk the value is around 40, if I move the phone 50 cm close to a small, portable radio with a tiny speaker, it increases to around 400, so it seems to be very sensitive.
Perhaps an app like this can help with troubleshooting electromagnetic interference messages.

ArcticPhoto Posted at 11-21 12:02
There is an app for it, of course :-)
Most smartphones have a compass (and therefore a magnet), and various apps can measure the electromagnetic field quite accurately. Normal value is between 30 and 60 microTesla, higher than that indicates that there is some kind of interference. I use the "ultimate EMF indicator Pro" https://play.google.com/store/apps/details?id=com.mreprogramming.ultimateemfdetectorpro (for android)
Right now at my desk the value is around 40, if I move the phone 50 cm close to a small, portable radio with a tiny speaker, it increases to around 400, so it seems to be very sensitive.

Thanks for linking to that EMF app.  very interesting.  

The compass interference reading in DJI Go 4 as measured by the aircraft itself would probably be more meaningful.

When all powered up and connected this can be viewed in "Main Controller Settings" / "Advanced Settings" / "Sensors" menu, and then tap on 'Compass' tab.

Strange that a watch receiving radio signals to keep time in sync would put off enough EMI to effect your M2P.  Usually EMI that would have an effect on something like M2P / RC is seen with transmitters, not receivers.

Aardvark Posted at 11-21 16:50
The compass interference reading in DJI Go 4 as measured by the aircraft itself would probably be more meaningful.

When all powered up and connected this can be viewed in "Main Controller Settings" / "Advanced Settings" / "Sensors" menu, and then tap on 'Compass' tab.

Believe checking drone compass for interference is wrong track here.  
OP's title and thread says Electromagnetic Interference (warning) - as in RFI.

ArcticPhoto Posted at 11-21 12:02
There is an app for it, of course :-)
Most smartphones have a compass (and therefore a magnet), and various apps can measure the electromagnetic field quite accurately. Normal value is between 30 and 60 microTesla, higher than that indicates that there is some kind of interference. I use the "ultimate EMF indicator Pro" https://play.google.com/store/apps/details?id=com.mreprogramming.ultimateemfdetectorpro (for android)
Right now at my desk the value is around 40, if I move the phone 50 cm close to a small, portable radio with a tiny speaker, it increases to around 400, so it seems to be very sensitive.

Interesting app. thanks for sharing

Wifi routers are the biggest culprit