Other mistake: When Clarice is reading the letter from Hannibal, we hear a voice over, however what he says differs from what the letter says, specifically the "your good friend" before the signature, which Hannibal says, but which isn't in the letter. (00:35:35)
Other mistake: Clarice handcuffed Hannibal. Apparently Hannibal opted to amputate his own hand, but the next scene shows officers descending on Clarice at the lake shore. Had Hannibal amputated his own hand, she would still have been wearing the other cuff. She wasn't.
Answer: As seen in the first film and in this one, Lecter demonstrates an almost supernatural ability for eluding the law and seemingly being in two places at once. This film was made in the year 2000, before airport security became extremely tight in late 2001. At that time (before 9/11), it was still possible to enter an airport's main concourse through the baggage claim or even from the tarmac without passing through rigorous security. As ingenious as Lecter was, he could have accessed the airport in a number of ways back then. Relieving another passenger of his boarding pass and identification would be no problem for Lecter, either (simply leave the passenger's body in an airport toilet and assume his identity). For the most part, it was Lecter's calm, self-confident charm that allowed him to slither through society always ten steps ahead of the law.
Charles Austin Miller