So when we look at this video.
First Hattie makes a low squeak like sound twice. When she does this, Aurora, the only other hen in frame stops and extends her neck. Sydney, not in frame did the same. I could not see anyone else.
Then Aurora gives the siren-like call loudly. Everyone stands tall and still. Aurora does not seem to be looking for anything. Her head does not move or scan anything that I can see but Hattie's head is twitching and seems to be agling upwards at times.
Aurora continues to make a low growl. Like Hattie, Sydney's head is twitching and she seems to be looking about.
At the minute mark, Aroura's head moves 45° to her left and she continues the low rumble. Could it be that she is tracking something while the others were looking for it?
At 1:23 Aurora's head moves back almost to her original position and she has given her last growl. Then she starts twitching her head, also looking around. All three of them are looking around until 2:30 when Sydney gives up watching and starts walking.
At 2:46 Sansa starts moving, then Aurora and Hattie start moving and the alarm is over.