If it did escape from the lab, I cannot help but think about how fateful of an event lead up to it...

A tech accidently knocking over a beaker? A tiny rip in a bio-suite thrown in a trash, missed the can, and picked up and put back on the shelf by another worker? A leaking containment vessel, known to need replacing, but still worked "good enough"? An animal infected with the virus, handled by someone who wasn't aware of what it had?

Endless possibilities, but likely something so small and seemingly insignificant.