The interviewer was probably just fishing for a reason for denial or trying to trap him in a lie. The poster said the trips were for his job ... if he was lying he wouldn't be able to provide proof. Similarly, even though it is not a requirement to be employed during the 5-year period (except EB applicants who should be employed for at least a few months post-GC), they still often ask for proof of that employment if you listed your jobs on the form. In other words, whatever question you answer, they may ask for proof, even if the answer is irrelevant to citizenship eligibility.
Yes, that could also be it as well, although this is the first time I have heard of somebody needing any letter or documentation pertaining to WHY they traveled outside the US.
What worries me is that I also have quite a few trips out of the country listed, in addition to writing on my N-400 form that I probably have additional trips that I simply cannot remember that may be longer than 24 hours. In my case, I live about 90 miles from the Mexican border and have made quite a few trips to Mexico since becoming a PR. Virtually all of the trips I have listed are "personal" in nature, meaning that they are completely unrelated to my job.
I am concerned that if they now start asking for the reasons for trips, or providing some kind of additional documentation regarding them. The "reason" for the trips is nothing more elaborate than the fact that I enjoy traveling and I like Mexico very much. Most of the trips were weekend getaways, picking up ceramics and whatnot for my house (I bought a fixer-upper here in souther cal) or just hanging out / surfing / enjoying the baja. I don't have a better explanation, and I certainly cannot provide a letter from anybody regarding the rationale or purpose for the trips - I took them because I wanted to, not because an employer told me I had to.
Here is what I listed on my N-400:
Total trips: 15 (none longer than 6 months in duration)
Total days outside the US: 58
longest trip: 8 days in duration
Plus, I made a statement that I made additional "day" trips to Mexico, most of which were less than 24 hours in duration, but that in some cases may have extended to more than 24 hours. I estimated 6 - 8 such trips, and no more than two days out for each one. Worst case scenario: additional 16 days out of the US over a 5 year period.
Should I be worried now or try to get additional documentation in place? I have letters from all my employers with my employment dates that supports my N-400 claim (I have been continuously employed in the US since I got my GC). I also have pay stubs and mortgage statements, so I think I can prove that I always resided here. I just never realized that taking trips could be such a problem, especially when the total number of days out is still relatively small. Any thoughts on what I should do to prepare?