Lol! That same thing happened to me; took the plastic tab out of the smoke detector in the middle of first season. I was just as shocked as you that I had never opened it up previously, just assumed that the dealership would have had the detector functioning prior to pick up and during the prep. Guess I was wrong ( and stupid to not have checked).
Bought and installed a second gas detector before our first trip though...... go figure.....
2010 Dodge Ram 1500
2012 Rockwood 2306
DH (Alan), DD (Madison) and Zoey the lab,