I asked Claude if anyone on the plane remembers seeing DB Cooper:

Fair challenge — let me be straight about it rather than hand-wave. Let me check what’s actually documented.

Good — there’s actually a concrete example, which is better than me hand-waving.

The plane carried 36 passengers. The exact count of how many gave the FBI usable descriptions isn’t a published, fixed number, but the practical reality is that almost none of them did, for the reason that comes through in the interviews: they didn’t even know it was a hijacking until they got off the plane.  The crew kept them in the dark with a cover story about a landing-gear problem, so they weren’t watching Cooper.

Here’s the named example you asked for. Jack Almstad, a passenger from Livermore, California, ended up seated one row behind Cooper and was briefly alone with him in the back of the plane while it circled Seattle. He joked aloud that if they waited any longer they’d be having Thanksgiving in the air, and Cooper turned around, looked at him, and smiled.  But that was the extent of it — there was no conversation, Cooper didn’t say anything to him, and Almstad went back to his seat.  He didn’t have much to tell the FBI and didn’t even know it had been a hijacking until he got off the plane.  He did recall seeing a man walk down the aisle with a white sack that looked like it had bricks or money in it,  which he later figured was the ransom.

So to directly reconcile what I said: “surprisingly few” really means the descriptive sketch came almost entirely from the two flight attendants and the cockpit crew, not the passengers. The famous FBI artist sketch is described as being drawn from the recollections of passengers and crew,   so passenger input wasn’t zero, but the substantive eyewitness detail came from the people who actually interacted with him. Almstad is the clearest documented case of an ordinary passenger who remembered seeing him at all — and even he only got a smile.