It happens near the end of the episode titled "A Technical Error." Sort of a nice inside joke, breaking the fourth wall. He whistles just a half dozen notes, but it's enough to identify the series theme music.
A person's a person, no matter how small. -- Dr. Seuss
Just heard him do the same thing in Trooper Maverick, but he whistled half the theme song! I'm binging in Maverick episodes all week while the wife is away!
shareBut let's get pedantic and imagine an "untold story" in which, among the numerous other recurring, colorful characters in the series, the Maverick boys also knew a guitar-strumming barfly who drifted from town to town and encountered Bret and Bart enough times to make up a song about them.
(I can't help myself, sometimes; I have a "fanboy" mentality that has to make logical explanations for plot holes and continuity errors!)
Secret Message, HERE!--->CONGRATULATIONS!!! You've discovered the Secret Message!
I love that idea, vinidici!
One of my own favorite hobbies is filling in plot holes and thinking up in-universe explanations for apparent discrepancies.