Looking over the NOUO's at the FCC site over the last few years, it appears the bulk of them are to pirate operations in the FM band. There are a handful in the 1600-1710 kHz range and only one I saw for a shortwave pirate (July 2011, The Crystal Ship station). I am guessing the FCC is not bothering anyone who doesn't have a complaint filed against them, i.e. they are not actively monitoring any specific frequencies for pirate operations. I would guess that in any area a legally licensed FM broadcaster is likely to hear about a pirate operation fairly quickly, especially if the pirate maintains a schedule and transmits more than a very short period in a day. They are the most likely to file a complaint to the FCC about pirate operations in their areas.
I remember when I was in college my dorm had a "dormitory broadcasting service" on FM which covered maybe a couple blocks and broadcast mainly elevator music. One weekend some of the students hijacked the transmitter and started playing music more to the liking of the dorm residents.