I don't think the loch-ness monster can be used for an equivalent comparison.
I understand the steps of logic you are making, and the reasoning behind, I just think the question is badly formed.
The problem with loch-ness, is that the lack of evidence of it's existence IS evidence for it not existing. This is because there have been many searches and studies to find it, and although we have no cold hard proof that the loch-ness monster isn't there, the lack of evidence brings it into doubt.
A fairer question should be, does the lack of evidence of UFOs give reason to believe they do not exist. This is an area in which there is not much evidence either way, and answers between that and, 'does God exist', are a lot more comparable.
See my post here.
I think you are making the same mistake as others; namely, assuming that the question is saying that any evidence for the existence of God must be tangible. The question is not saying this at all.
The question tacitly accepts that existence of God does not hinge solely upon tangible evidence. Evidence can be acceptable even if it is not tangible or even wholly objective - as I have demonstrated by reference to the Battle of Kadesh.
The question is simply testing the consistency of your methodology. If your methodology is logically and rationally consistent, you will pass the question. If it isn't, you won't.