When Princess Leia Organa disguised herself as a bounty hunter to rescue Han Solo from Jabba’s Palace in Return of the Jedi, she didn't know the Tatooine crime lord was aware of her plan - but how did he discover it? Return of the Jedi finally saw Leia and her Rebel friends come into conflict with the gangster, to decidedly mixed results. Their plan was far from perfect, but it turns out someone already inside the palace advised Jabba something strange was afoot.
After claiming the reward for Han’s delivery, Boba Fett continued to work for Jabba the Hutt and was present in the palace during the final chapter of the Star Wars original trilogy. Donning the armor of bounty hunter Boushh, Leia pretended to cash in on the bounty on Chewbacca’s head, supposedly earning the respect of Jabba and even Fett, who nodded to her in admiration. However, Leia’s rescue was short-lived when a curtain was pulled back to reveal a supposedly empty room was secretly hiding Jabba and numerous members of his court.
According to the movie’s canon junior novelization, Star Wars: Return of the Jedi: Beware the Power of the Dark Side!, Fett told Jabba that Boushh was not who he claimed. His reason for expecting treachery was sound and makes the Rebels look foolish. Even if Fett hadn’t alerted the gangster, Leia and Luke Skywalker's plan was strange enough that Jabba surely would have suspected an ulterior motive.
Fett knew it couldn’t be the real Boushh because that bounty hunter was dead. Before the events of Return of the Jedi, Leia was attacked by Boushh on Ord Mantell, where the Rebel leader disarmed her attacker and, on the advice of Maz Kanata, stole his armor to help in her mission to save Han (as shown in Star Wars Forces of Destiny). Shortly afterward, Boushh was killed after he attempted to blackmail the Black Sun crime syndicate. While Leia hoped word of Boushh's death hadn't yet spread across the galaxy, it makes sense someone as infamous as Boba Fett would keep track of his competitors' actions. Therefore, before Leia reached Jabba's throne room, Fett informed the gangster that it would not be Boushh under the helmet. Jabba assumed their visitor was a smuggler in stolen armor - the revelation of Leia was merely a bonus. This retroactively brings a new perspective to their scene together, with both Jabba and Fett silently sizing up the masked figure, already aware of a potential threat.
As one of Star Wars' most powerful gangsters who often dealt with bounty hunters, there was already the possibility that Jabba knew Boushh and would have suspected foul play. This is just one of several flaws in the heroes' bizarre plan. It's unclear why the group entered the palace separately and Luke's hologram message to Jabba, delivered via R2-D2, clearly indicated Luke wouldn't abandon his friend Han. Jabba appeared to dismiss Luke's warning but he would obviously be cautious of the Jedi's impending arrival. For "Boushh" to suddenly appear with an overly compliant Chewie would only arouse further suspicion. The Rebels should have known there was no way of leaving the palace without a fight.
Boba Fett had previously battled Luke on Tatooine and, briefly, on Cloud City, so he was well aware of the Jedi’s powers and neither he nor Jabba would be so naïve as to think there wasn’t a plan in place for Han’s rescue. While the plotting and motivations were dubious, at least the sequence provided a memorable glimpse into life at Jabba's palace. Even without directly addressing Jabba the Hutt onscreen in Return of the Jedi, Fett remained an intimidating presence by keeping a watchful eye over proceedings. The novelization merely enhances his cunning nature.
