It's amazing how in tune with the force Obi and Yoda are near the end of the movie ... they couldn't tell that the chancellor was really Darth Sidious for many years, but they instantly start refering to him as the Emperor at the same moment that he announces the Empire ....
obi wan changes the encoded message then as he and yoda are leaving he says there's something he has to know and brings up the security camera footage. in it Palpatine tells Vader to bring peace to the Empire. not a stretch for them to figure out palpatine is the emperor of this new empire
