As mentioned in my original comment, if someone isn’t comfortable showing me the source code I simply ask for them to describe how it works, challenges in building it, how they built it, etc.
I could do this and would welcome this type of interview experience. I'd be able to talk in general terms about the problems I work on and their solutions but not the low level specifics.
To add some context, my work is in Financial Services, trading systems and whatnot.
What? Yes. I expect I'd be in prison for a few years. Your interview approach selects for people who don't follow their NDAs
Sergey Aleynikov went to prison over copying GPL code.