Magnetic anomaly detectors are already used by many navies for submarine hunting and minesweeping.Typically mounted on aircraft. They use to use similar equipment in the Vietnam war to detect truck ignition coils.
So quite possibly the magnetic field could be detected. On the other hand,if you're transmitting radio, then you can be detected. And if you had an normal antenna, that also sticks out like a sore thumb on radar. It took a LOT of work to get the radars on the stealth fighters not to light up like a Christmas tree. If you could set up your transmission to be fast enough (burst transmission), then your magnetic field would be up for too short of a time to be useful (possibly), much faster than a mechanical antenna can extend and then retract.
Basically, I'm sure they thought of that. War is their business, they tend to be pretty good at it.
If not, that's pretty neat. Now to make an iPhone app for it...