It is the Prisoners of War / Missing In Action flag.
Aka POW-MIA. It is to honor soldiers who were either captured by the enemy and held in prison camps, or who were missing in action, meaning the soldier "went missing" and it is presumed they were killed and their bodies were not able to be recovered from the battlefield.
The flag came into popularity during/after the Vietnam War. In that war, the enemy provided poor documentation of prisoners of war, so "missing in action" often meant "prisoner of war".
As far as how mainstreamed it is, I am reading this at an Iowa rest stop, and as I came in, I noticed the flag was
The upshot is that it's not uncommon to see the POW-MIA flag paired with the US flag. In this case the state of Iowa flag is there too, no surprise since the state owns and manages the site. And, obviously, made the editorial choice to include this flag.