as a former fly-by-night contractor I can tell you that its not uncommon to meet with customers after usual hours especially with cash only customers who want to keep a low profile. It is also common to do the work in the evenings, overnight, or weekends when there is no building permit being issued. I wouldn't be worried about it.
For that matter, I may stop by and see if I can barter my services for theirs. It could happen.