Final Resolution! Reace confirmed that there is indeed a spark - the propane
detector draws enough current to trigger a spark. Unfortunately, the propane
detector must be installed without a switch. So, my choices were either (1) ignore the spark (which I'm sure most people do, but all the advice about not having sparks near the battery
scared me away from this), or (2) remove the fuse for the circuit that the detector was on. I did (2) and everything is now fine. I have all my limbs, my eyesight, and a trailer ready to go after 7.5 months of winter.
Given all the responses and lack of similar problems for others, I suspect that very few people actually disconnect their battery