propane smell

We heat our home with propane and we notice a slight odor of ethyl mercaptan when our tank is near empty.

The odorant used wirh LPG is a much heavier molecule than propane, does not vaporize as readily, and will become more concentrated as the fuel is used up. Your nose is sensitive to the molecule in very small concentrations (why it is chosen as odorant for consumer fuel gases) and you'll notice it more in unburned gases as fuel volume in tank goes down.

I don't understand why, with the furnace exhaust gases, because they are vented outside. Suspect some of the furnace exhaust is seeping inside (slideout rooms maybe?) . Not enough to be pickep up by CO detector, but enough for the percaptans to be noticed by the detector on the front of your face :)

Yes. I was told that the furnace was getting some propane but very little and not enough to ignite. It concerned me enough to wonder why the alarm didn't go off but strong enough for me to smell it. Happens to me more when temps are cold, but always when propane is almost empty.
Did test on alarm per instructions and it was OK.

GF could always tell when propane was getting low......but it was when using stove top burners.

Propane smell inside from furnace......shouldn't happen----LP Detector should be alarming
Propane smell outside next to exhaust......slight odor
Propane smell at cylinders....slight odor

The additive (ethyl mercaptan) can be more concentrated with low propane level.