Assuming that the candidate was extended an offer and asked to look at the codebase, I would require an NDA. I work in a niche market so you can't get the secret sauce in a sit down, but there are methods that can give you an advantage. For example, I worked at a place that did something they weren't supposed to be doing to gain a competitive advantage. To this day, 8 years later, despite working at several of their competitors, I still haven't seen anyone else do this thing. I haven't revealed this to anyone and I won't. We aren't doing anything like that, but we do have methods that I haven't seen till now.
