A report of a suspicious person crackled from John Messner’s RadioShack police scanner, one of two he keeps at his home in Knoxville, Tenn.

When an officer was heard yelling “Shots fired!” minutes later, Mr. Messner knew it was time to go. The 52-year-old construction worker and photographer grabbed his two cameras, his portable scanner, jumped in his 1999 Plymouth Voyager minivan, and raced to the scene 3 miles away, where a suspected burglar was shot by police.

“When...