>There's no way to send a 2mm (width) wave.

You are very wrong sir. https://en.wikipedia.org/wiki/2-millimeter_band

>A helicopter with the right equipment flying over urban areas would have an easy job finding them.

That is not true. Helicopter would need to fly RIGHT into such directional and tiny beam. Which is nearly impossible to do in huge city. Even when flying 2m next to it, you would likely not detect it.

That is the reason why CIA had so much problem with pinpointing Iridium satellite phones back in Colombia in the 90's.