That's a difficult question because then we have to define 'scientific' and this might never end. LOL.
I think to say "I believe X" when X cannot be categorically proven is necessarily to make a leap, to inject a degree of faith, no matter how seemingly small, and that there is no legitimate definition of "believe" in the English language that can remove this element of faith. Is that "unscientific"? I think it's contrary to strict principles of good science, and yet it is very common. So common, in fact, that someone who insisted on such accuracy would be something of an oddity as a scientist. But they would not be an oddity as an agnostic.
With whatever definition of "believe" you use, either as indicating acceptance as true because of perceived high probability/improbability or acceptance as true because we believe something to be absolutely true (or false), both require degrees of faith to reach "acceptance". Faith and belief are not - for me - part of good science, but I fully accept that they are operative within science (even if this is not always readily admitted). That makes them 'scientific' by virtue of being endemic within science and among scientists, but not 'scientific' if one believes good, pure science, involves being as accurate and unbiased as possible.
To sum up what I'm saying as simply as possible: no matter what definition you use of "believe", if you say "I believe X does/does not exist", you are a "believer", and if you can't categorically prove 100% that which you believe then your belief involves faith.