Even as a US citizen I'm sometimes surprised at the kind and number of questions you can be asked when entering the US. In late May we returned from a month in Europe via CLT. When we got close to actually dealing with an agent we could hear one agent quite clearly and while I would hesitate to call it a "grilling" he was asking everybody who went to his station things like "where have you been?", "why did you go?", "how long were you gone?", and so on. So we actually talk quietly about our answers thinking this was SOP. We go to the agent next to the one asking all the questions and all we get is "Welcome back. Have a nice day."