If it
can happen in a lab studying an existing type, it
can happen in labs studying GoF. The 1977 pandemic was a "leak" of sorts that seems to have originated from a prisoner camp known to give reduced sentence times to inmates willing to participate in medical research. The "leaking" doesn't have to happen from a vial or mouse, the avenues are numerous (Marburg "leaked" when African green monkeys, imported for non-viral research, were handled incorrectly without infectious precautions).
> But of an already existing virus, so not so bad.
Millions of drivers every day. Hundreds of accidents. Many quite light. Still, the presence of light accidents outlines the possibility of heavy ones. And that's why "phew, got lucky this time" should lead to better protection not a "not so bad, nothing happened" attitude, IMHO.