The most plausible reason to me would just be the fact that it's DHS doing the searching, not FBI. Border control has very broad authority to search and questi

The most plausible reason to me would just be the fact that it's DHS doing the searching, not FBI. Border control has very broad authority to search and question anyone coming into the US, and some office at DHS may be very interested in her but the equivalent office at FBI might not have the time or inclination to pursue it further, especially if the searches and questioning keep turning up nothing. I worked in the military in the past and have seen plenty of examples of government organizations with different but slightly overlapping missions. They tend to step on each others' toes just as often if not more often than they work together effectively. I also have no idea if the FBI pursued it further and got a warrant. They're under no obligation to inform someone that they're under investigation. They may very well have started up a case against her that's still open, or may have investigated her and eventually dropped the case, or may not have done anything at all. In any case, they've never charged her with a crime and she's never been arrested.