Re Spotting the Boomer.

Defiance was at a different angle, she was already in "active" mode while Nene
was still in Passive, and Defiance just happen to be "looking" right at it when
it started to power back up.
Also Nene might have been distracted when she picked up Defiance's search beam.

If Defiance hadn't opened up when she did Nene would have spotted it in time to
warn the other Sabers