Alice has made some anecdotal observations. Through a process of elimination, she proposes a hypothesis to explain the phenomenon, as well as an experiment to validate (or otherwise) her hypothesis. The experiment has yet to be performed.
Bob largely agrees with Alice's deductions, however he advises that there may be other factors at play. When challenged on this by Alice, Bob mentions things that (unbeknownst to him) Alice has considered in her elimination process. Regardless, Bob still believes that it is reasonable be open-minded, even though he cannot immediately offer an alternative hypothesis.
Is Bob flouting the scientific method? I believe that playing the role of sceptic is both valuable and necessary -- please correct me if I'm wrong -- but does the fact that he cannot provide a tangible alternative relegate him to quackery and pseudoscience?