https://www.youtube.com/watch?v=JzdnUZReoLM

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...