> the longer time that passes without finding a natural source outside the lab the more likely it is that it was created in the lab.

This is not correct. It may well be that the evidence existed but was lost.

The idea that biology will just sit around and wait until we catch up with it is fundamentally mistaken. Some things happen just once and that's that. Some things happen all the time and you can observe them as they happen. Some things leave ample evidence. Biology is not a static system, it isn't a computer program and it isn't a piece of hardware. Your 'bug' may simply not be reproducible even if it in fact did occur just once.