So the next morning, I'm shoveling snow ... and my neighbor to the West says
"Hey, I want to show you something"
As seen in the pictures below, a bullet hit his house - YOWSERS!!!
He explained that about 2:30 in the afternoon, they heard a BANG ... and assumed a picture had fallen or something in the house. After looking all over and not finding it, they moved on. That evening, he noticed the damage next to his front door ... and then a rifle bullet laying right underneath it! He called the police who made a report and took the bullet - said either a .308 or 30-06.
As seen in this YouTube video, I was in my front yard less than a 100' away shortly after the bullet hit - D'OH!
There isn't much they can do to track down the shooter and shooting is legal in the unincorporated land to the NE of our neighborhood ... although unsafe shooting can result in a reckless endangerment charge. Since the shot almost certainly originated on Boulder Country Land, he also filed the report with them and both police forces will keep an eye (and ear!) out for more shooting.
Hopefully this was an isolated incident - we both agreed it would be a terrible tragedy if a person was hit by a stray bullet.