When I travelled to US as a tourist (3 visits of 1-2 weeks each) on ESTA, I once got a question from CBP. It was a friendly, "casual chat style" question, the guide almost smiled at me saying "Are you working here buddy?" to which I replied firmly I wasn't, as I had a full time job back home. I travelled from a very trusted European country, but I later realized they're probing me I guess...