https://chromium-review.googlesource.com/#/c/335584/ introduced this line:

  killall -9 frecon

Unfortunately, killall is only available in dev and test images, but not in release images, so this line isn't effective at all.

Note that this can easily be reproduced even on a test/dev image by going through a powerwash:

  echo "fast keepimg" > /mnt/stateful_partition/factory_install_reset
  reboot

Then:

root@localhost ~ # display_low_battery_alert
/sbin/display_low_battery_alert: 42: killall: not found

frecon still starts as the failing killall invocation is ignored. I suggest adding "set -e" at the beginning of the script to switch the shell to complaining loudly to prevent problems like this going unnoticed.