Schiphol Airport has large No Balloons signs when you go down to the train station. Aluminum balloons can create havoc on the overhead power lines. It recently shut down the train service for the morning.
How have we not blown ourselves up yet?
It's not that we haven't, it's just that we can only observe from those few realities where we didn't.
Most people aren't sadistically malicious, and most security is professional, so random little failures like metalled balloons or reflections off clouds (or the moon) scaring a security system will only blow up to something important (even on the scale of previous paragraph) every decade or so.