At 16:28, on the plane, Shelley Winters' character calls Gary Merrill "Mr. Trask." How can that be? No names were mentioned during introductions in the airport cafeteria, and Gary Merrill was travelling under an assumed name (Collins) and even evaded a phone page there. Even though his briefcase said Trask, it's doubtful Winters' character would have noticed it. It really caught me by surprise, especially since Trask had made it a point to travel pseudonymously.
You must be very young! Kidding aside, there's a simple explanation. Before they board the plane, when the four meet at the airport cafeteria, Binky tells them about a show where she performed recently and she wonders whether the doctor has seen it. He has but can't remember, and then the lawyer says "I remember, I watched it with Mrs. Trask." IOW, he revealed his name! Even though only a few minutes earlier he ignored the PA system when a Mr. Trask was paged for a phone call at the ticketing counter. So, Trask did reveal his real name to the group early on.
Moreover, the lawyer has what looks like a leather bag or carry-on with his name, TRASK, which is clearly shown when he first arrives at the airport. Binky may have seen the bag too, before we got to the 16:28 time mark.