They hold some form of belief as regards the existence of flying saucers, you just don't know what it is yet. Maybe 'they exist', maybe 'we don't know' or whatever else. They don't lack belief as they are aware of the concept of flying saucers.
It is perfectly logical to not know whether flying saucers exist or not. This does not mean I have to have a
belief about flying saucers. It just means that I am
aware of the concept of flying saucers. A belief is
held, in the absence of absolute and complete knowledge. If I choose not to believe a claim until proven, my lack of belief is in itself not a belief, it just means I am skeptical.
Belief and knowledge are two very separate concepts, and I think you are having a hard time distinguishing between the two.
As soon as someone becomes aware of a god concept, they hold some kind of belief about the existence of god(s) that will put them into one of the atheist/agnostic/theist/etc. camps. A lack of belief is not possible, it is simply an absence of something. If you can comprehend the meaning of 'do god(s) exist?' You have some form of answer, some kind of belief as regards the existence of god(s): 'Yes, of course'; 'I find it improbable', 'that question can not be answered'. What you don't have though is the absence of a belief, a mental vacuum, there is something there no matter how small and unconsidered.
That would be like saying that as soon as you are aware of the concept of a stock market, you have to have a belief about how a particular stock is going to perform tomorrow.
Let consider this:
Person 1 : Stock A is going to increase 2000% tomorrow
Person 2: I don't believe you.
In this situation, does Person 2 need to have any belief about Stock A's performance? Why is "I don't know" not a good answer in this situation? Why is Person 2's lack of belief in itself a belief?
Person 2 simply doesn't have enough information to have a belief about a particular stock's performance. You may form an opinion or a belief after following the performance trend of a particular stock for a long time, but it is not reasonable to expect someone to hold a belief about a stock either way just because they are aware of the concept.