>That would imply that all atheists are omniscient. In order to know for sure that something does NOT exist, you would need to know all...

What you're describing here is only one type of atheism that exists, and admittedly, its just as bizzare as theism IMO. There are other atheists (and this is typically confused with agnosticism) that don't believe that there is no god, and just don't know. There is no reason to suggest either way, so why is it even a question, worth debate?

This isn't to say that they believe that we'll never know, or that its impossible to know, which is the acting definition of agnosticism. Most atheists I know would also be puzzled by such a belief when no evidence suggests that conclusion either.