These are the CIA’s tips for spies on how to avoid detection at airports (qz.com)

I always been asked these questions every time I travel from China to US and vice versa, like "How long have you stayed there?" and "Why were you in China?" If you don't wan't to be spotted on from the CIA, just pretend you didn't do anything wrong and act as if you were just a visitor exploring the country. Better to sound confident and not give a shit what others would think. You would save yourself so much more hassle like that.