It's supposed to be that electronic gadgets' emission are dangerous for airplane systems. If this is true, why don't they use any detectors to locate such emissions?
Three reasons I can think of:
It's easy to detect the general presence of a specific transmitting device (a WiFi antenna, say), but very hard to precisely localize it. They could tell if there's one on the plane, but someone with a very sensitive piece of equipment would have to walk up and down the aisles pointing it at people to actually find it in somebody's pocket. And cellphones (when not in use) are generally burst transmitters: they don't broadcast continuously, they just "ping" the tower every so often. This means it could take a long time and a lot of aisle-pacing.
Many "electronic devices" which are banned are entirely passive and have no traceable[*] emissions. Think cameras, handheld GPS receivers, iPods. (On the other hand this passiveness also means they are extremely unlikely to harmfully interfere with the operation of the airplane, but right or wrong, such devices are still banned in some cases.)
It's not that important. There are a few incidents where electronic interference is suspected as a possible factor, but no conclusive evidence. As soon as there is actually a fatal accident where an electronic device can be conclusively blamed as the cause (and I doubt this will happen), the airlines will be a lot more willing to spend the kind of money that would be required to implement this.
For a full, evidence-based discussion of the actual risks, I refer you to this question on Skeptics Stack Exchange: Are personal electronics a risk to commercial aviation?
[*] No traceable emissions. All electronic devices generate some very, very tiny incidental signals. But these are so weak that they will be virtually impossible to detect on a crowded plane filled with its own (approved) electronics. Such emissions are measured in laboratories with extremely sensitive antennas in special, heavily-shielded rooms under carefully controlled conditions.
One thing to point out, electronics on planes really isn't as big of a safety factor as the airline would have you believe, and it's becoming less and less of an issue since consumer demands are pointing towards the aircraft that allow them to be entertained while traveling, such as have WiFi.
For the companies that run the phone lines it's a different story entirely. If your careening through the air at 500 knots and you make a phone call, you require the phone towers to do a lot more work receiving your call. I'm not sure why, but I remember reading that the main movement behind not using electronics while flying was really because our telephone infrastructure isn't capable of handling the excess load.
Due to plentiful aircraft shielding and low power emissions of personal electronic devices, practically speaking only a malfunctioning electronic unit that is transmitting the exact same frequencies that the antennas are designed to receive and filter out carry significant probabilities of affecting avionics. One hundred iphones with wi-fi turned on would almost always have an effect, as they would not be emmiting signals received by the antennas. (If there was a rare malfunction a signal could pass from the cabin, through the window and into a GPS receiver, for example).
To identify the position of a malfunctioning unit one would need expensive multiple receivers and a computer that triangulates the location. This is just too expensive for the low risk.
Even if you could identify a malfunctioning unit, the unit would notmyet be malfunctioning otherwise the aircarft would already be receiving the bad signal and in a sense the aircaft systems are the first detection.
On the whole the answer lies in the fact that the interference for the odd electronic, people forget to turn off is low risk and does not justify the expense of an advanced locator system. It is the aircraft that provides the saftey by properly shielding its electronics from interfering signals to begin with. The antennas restrict communication on narrow frequencies amd usually with certain communication protocols, so even at the right frequency disturbing the aircarft instrumentation or navigation is a freak accident. I think the most probable interference is that a malfunctioning noise transmission across various uncontrolled frequencies could distort the coding a proper signal trying to communicate through the antenna to a given receiver.
The greatest danger appears to be when an aircraft is flying slowly, with landing gear open and there are cell towers on hills level with the approach path, because these are circumstances when a cell phone is most likely to make a call connection.
Many years ago working as an airline baggage handler in a country which prohibited all cell phone use during commercial flights, I was astonished to hear baggage carts with phones ringing all the time and it is the airport baggage halls where one could implement this policy or at the check-in counters. I imagine airlines would not be too keen on the extra manpower and wages required.