Are you sure it was USB? That is in the 75 meter Amateur band(and usually LSB) and you likely heard one of the National Park On The Air stations. Hams are 'activating' various National Parks by operating portable from them. Have heard many contacts with some one saying thank you for activating ____. with the black being a National Park or Monument. Many hams are trying to contact all the parks activated. You can get more information at ARRL.org.
Could it have been 3.996MHz USB? That is a common amateur military radio collector frequency used across the US. There is a west coast net on this frequency every Wednesday night at 8pm pacific time.