Verbally agreeing to the NDA then not signing might count. Signing a fake name would obviously count. Editing a digital contract before signing might count, and not highlighting the edits in a "track changes" negotiation would probably count. If you returned the papers and said "these are all set" or something, I guess you might be at risk.
But just... not signing a thing? If someone puts it in front of you without negotiating or asking for consent, then accepts it back unsigned? I've never heard of a lawsuit arguing anything even close to "I handed him a paper, so he should have spoken up to tell me he wasn't signing it", and I can't imagine it'd go anywhere.
(On which note, every NDA I've seen has been explicitly brought up to ensure that I was ok with it. But I guess that's not universal?)
If they offer me a separate contract to sign for their non-disclosure terms and I agree neither verbally nor in ink, I think it's a stretch to call it a misrepresentation of my position. Given that this type of contract usually offers absolutely nothing in exchange, it would be more reasonable to assume that I don't agree, if I don't sign it or express agreement otherwise.
Also IANAL, but at the very least I don't think that this is unethical.