Really ones I have seen you would have to enter through arrivals which would look mighty suspicious walking the wrong way up nothing to declare
But maybe I have just not been paying much attention to how open they are.
For domestic the belts are usually directly in front of arrivals, so anyone can just walk in and mingle. There's no "nothing to declare", just doors to the street. People come and go all the time to meet family from flights for example, there woundn't be anything suspicious about it.