It's usually things like - where have you been? For Xmas? How long were you there for? Where are you going next? Etc- like small talk but today I answered a question for where we had been, and then didn't hear his second question about how long we had been away for and he asked it again very seriously with some more follow up questions.
What sort of things are they looking for or to find out? If you'd smuggled a child back into the country then the questions aren't enough to suss that out, and if you weren't allowed in the country surely that would flag up on your passports?
Always wondered and thought a mumsnetter would know!