Detecting radio signals is trivial. Pinpointing those signals is the real issue. At best they could use a directional antenna, which would let them know roughly what direction the signal is coming from. Even then I'm not sure that it would work well because large chunks of metal (e.g. the airplane body) tends to distort those signals. Do a search on antenna patterns and ground plane to get a rough idea of what I'm talking about.

I'm guessing that the radio transmitter, and perhaps receivers, are the only aspects that they are worried about. Electronics do tend to spit out a lot of radio noise, even if they don't have a radio transmitter, but that should be relatively low power (especially for portable devices). Again, I'm not an expert in this field so I cannot be certain, but I do have a background in physics. If you are interested in this sort of thing, you can get an introduction to how electricity and the electromagnetic spectrum works by looking at a grade 12 physics textbook. There is also a lot of accessible materials produced for radio amateurs which will give you a grounding in what's happening.