Every weekend. Every. Single. Weekend. I "commute" to RI from NY most weekends visiting my girlfriend at Brown. Sunday night, when I head back, sometimes leaving as late as 11PM, I am guaranteed to see at least 3 people get pulled over in the dark. That's up & down I95, either direction, any mile marker. I've had plenty of cops turn on their lights just as I've gone past them, only to see them race after someone else.
So, yes, I've seen cops use radar at night.
A lot.