Was it always plugged in to AC power OR did you just plug it in when getting ready?
Battery shows 'full charge'.........those panel indicators will always show full charge when plugged into AC power.
If you just plugged it in when getting ready......agree that low battery voltage was cause of alarm and then as battery voltage increased from charging alarm reset.
This is a combo unit (CO & Propane)?
Always thought 'propane' detectors were installed down floor level and CO detectors higher on wall due to the way the vapors collected......hummmmmmm :H
I have seen combo CO/Smoke detectors but not CO/propane detectors.