Do you have a pilot light? If so, I'd think that during a cold spell, when there was very little propane left in the tank, the pilot light went out because of low pressure. Then during the day, when it's warm enough for vapors of propane to vent out the pilot light tube you get a whiff. It's noticeable but does not often cause the LP leak detector to sound.
If you have electric igniters, well, then it's probably the grey tank smell coming up through dried out P traps.
Just saw your last post...kinda sounds like a venting battery.