Warning: Knives Out SPOILERS ahead.
Writer-director Rian Johnson reveals a hidden clue to the killer's identity in Knives Out that many viewers may have missed. After helming 2017's Star Wars: The Last Jedi, Johnson returned this year with Knives Out, a riff on classic murder mystery whodunnits with an all-star cast. Daniel Craig leads the Knives Out cast as detective Benoit Blanc, who's hired to investigate the mysterious death of Harlan Thrombey (Christopher Plummer). All of Harlan's family members are suspects, and Blanc recruits Harlan's nurse/friend Marta (Ana de Armas) to help him unravel the mystery.
The killer is revealed in the Knives Out ending sequence, which features Blanc putting all the pieces together and explaining them to Marta, Detective Lieutenant Elliot (LaKeith Stanfield), Trooper Wagner (Noah Segan) and Ransom Drysdale (Chris Evans). As explained by Blanc, Ransom wanted to have Marta kill Harlan, and switched his medication with morphine to do so. He then called in Blanc to investigate so Marta would be held responsible for Harlan's death and she wouldn't receive the Thrombey inheritance. Though the killer's identity and plan in Knives Out aren't fully revealed until the end, Johnson leaves plenty of clues along the way.
Some of the clues to Ransom being the killer in Knives Out include the way the dogs react to him, Great Nana (K Callan) mistaking Marta for him and Fran telling Marta "Hugh did this" (though Marta mistakes it to be "you did this"). Still, there's another clue that Johnson points out during his in-theater commentary for Knives Out. During the scene in which Marta brings the blackmail note to Ransom's house, there's a clue hidden in the background. Johnson explains:
Also if you take a look in the background, notice there’s a jug missing from the line of jugs on his shelf. And that is because he throws one of those jugs through the window in the end sequence to burn down the medical examiner’s office.
When Marta visits Ransom's house, Ransom has already used the jug to throw a molotov cocktail into the medical examiner's building, but viewers don't know that until later. During the Knives Out end sequence when the murder mystery is unraveled, Johnson features the full shelf of jugs in two shots: Once off to the side when Ransom is putting cash in an envelope to hire Blanc, and again when Ransom receives the blackmail note from Fran (Edi Patterson). Both of these later shots feature five jugs on the shelf, with the latter scene especially calling attention to them. However, in the earlier scene of Marta at Ranson's house, there's one missing from the shelf, as seen in the above image from the set of Knives Out.
It's likely that most viewers missed this particular clue, though, since the empty space on the shelf is only visible for a very short time in the movie itself. Johnson explains in his Knives Out in-theater commentary the scene was originally much longer, but he ended up cutting down the "expository linking scene" to be as short as possible. So the missing jug becomes a hard to spot clue in the final cut of Knives Out. Since the scene was originally longer, and would've given viewers more opportunity to see the missing space on the shelf, the jugs were presumably envisioned as a much bigger clue.
While this clue is difficult to spot, it points to how well-crafted Johnson's killer reveal is in Knives Out. There are plenty of clues to Ransom being the killer, so even with one reduced to a hidden clue, there are even more paid off in the Knives Out ending denouement. Still, those who spotted the missing jug are now vindicated by Johnson - and those who didn't spot the missing jug now know one more clue hidden in Knives Out.