In these cases, I suspect that an FA either saw a pax using a cell phone, or saw a flashing light in a seat pocket, etc. and advised the cockpit, and that was their "indication."

I don't believe that radio detection equipment is standard on most commercially operated aircraft.

In order to prevail, you'd have to show that you were damaged in some way, which as multiple studies by aircraft, avionics and cellular equipment manufacturers have illustrated, you can't.

Greg