Propane, butane, etc. are not explosives. They are not going to just energetically decompose. They are combustible, but it turns out most things worth having on earth are combustible
https://www.youtube.com/watch?v=uJJvG10tUAw
They're pretty explosive when they have the right mix with air. This can easily happen in, say, a utility closet with a water heater or air handler, or a fridge in a kitchen, etc.
A small leak on the outside unit probably isn't going to be a big risk. A small leak on a unit in a closet with stuff that sometimes makes small sparks...