How likely is it that unpackaged, assorted batteries stored together will actually explode?

I have a bad habit of putting all of our loose batteries in a zip-lock bag; all different brands, sizes, etc, and without anything keeping them from touching each other. I’ve always done this without a second thought, despite the warnings I’ve seen on packages about it; probably because when I was growing up, my mother always had a “junk drawer” that consisted mostly of loose batteries.

How dangerous is this, really? What is the chance that the batteries stored this way will actually cause an explosion, leak, or do anything else that could be dangerous? Does there have to be specific environmental factors at play for this to happen?