Implicitly, when you print your visitor badge you consent to the NDA. Though that may be a generic site NDA, not an interview one.

I never had to provide an actual signature, aside from a verbal agreement with the recruiter to not share interview questions.

We make our candidates sign an NDA before an on-site interview, but do so because of the risk that they see/hear some of our internal information during the interview process not to entrap their ideas.