What I'm referring to is not the loud "Bok-bok-bok-bok-bok-BAWK!" repeated over and over at high volume, but the quiet, erie, intimate-sounding five note phrase, "Bop-bop-bop-bop-bop" uttered almost under their breath so as not to call attention to themselves, but still loud enough everyone in the flock hears it. Usually it's the rooster who calls it, but not always. Whichever chicken first notices the intruder will speak up, perhaps more than one.
When they see me coming, it's the five-note phrase, "bop-bob-BOP-bop-bop".
If you want to hear this phrase, invite a friend to approach the run whom the chickens are not familiar with. Or better still, a strange dog. It's usually uttered at first sighting of the intruder, preceding the alarm call that you're thinking of.