They're more common than they should be it seems. We have one here in the Las Vegas area at 167.3625 that people have wondered about for a long time now, on very rare occasions traffic will be heard in the clear, at other times it'll be encrypted P25 / NAC167 so it's FBI related at the bare minimum.
Guess the Feds just like keeping their repeaters on and warmed up 24/7, who knows.