This has confirmed an experience I had recently. I was flying near Brisbane CBD, over 6.5kms from Brisbane Airport but in proximity to a number of Helicopter Landing Sites. In Australia, you are permitted to fly here, so long as you land immediately if you become aware of an aircraft in the vicinity. During the flight I suddenly received an alert on my screen to land immediately as there was another aircraft in the area. Visibility was effectively unlimited except for some tall buildings, clear sunshine, no wind and no apparent aircraft noise. I decided to keep flying but about 10 minutes later, received the same alert again and landed. There was still no sign of any other aircraft. After carefully reviewing all my photos, there were two pics of commercial airliners as specs in the distance and then there it was, a helicopter, between buildings, over a kilometre away!
So, how did my DJI GO4 app know this? Only one possible answer, my app was communicating on a common band with other aircraft! This would only be possible if you were flying without your phone being in airplane mode...