You can prove there is no purple zebra on earth by actually surveying the population of zebras which is finite.

How do you know the one purple zebra wasn't just walking around in a way that meant they were always not where you were looking?

You can probabilistic say "it's extremely unlikely purple zebras exist" but you can never prove 100% they don't exist. And back to the real example, how can you prove there isn't a bug you just haven't found yet?