What? There's no way to send a 2mm (width) wave. Yes, parabolic antennas have very high gains in one direction, but they're far from '2mm' and far from 'invisible'.

A helicopter with the right equipment flying over urban areas would have an easy job finding them and even pointing out the approximate location (eg. a building) where they're located. If the antenna was inside a huge apartment building, it would be hard to find the right apartment, if the transmitter was turned off, but with single family buildings, it would be easy.

Ham radio operators hide and then look for hidden transmitters for fun, so it's not that hard, especially with a budget that is given to the police.