Big thanks to David who has been tracking this truck in Arlington since last week.  The truck is named Salt And Papper Grill (notice that Papper has an “a” not an “e”).  According to their website, they serve in Rosslyn (Mon-Wed) and Crystal City (Thurs-Fri).  And, according to David, the food is pretty good!  A scan of the menu and more info can be found on their page.